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topic 12453

Home anodizing-Truths from the experts?


Q. I am attempting to anodize some small aluminum paintball parts at home. I have found several "how to" articles, but they seem to have conflicting information on certain issues. I have questions that I hope someone can help me with.

1. What is the correct amount of battery acid to distilled water for the bath? (I have read 50/50, 10%, and 2/5 ratio)
2. I have a 20 amp HAM radio 12v power supply, and a 2/12 amp battery charger [linked by editor to product info at Amazon] . Which would be a better power source?
3. What is a good paddle design for the negative lead? (I have read: A circle of aluminum wire covered with aluminum foil, wire mounted vertically in the tank, a flat piece of aluminum that runs the circumference of the bottom of the tank.

I am having problems sorting all this out, so I figured I would ask the experts!


Wayne Newberry
- Clintwood, Virginia


A. Hi Wayne.

1). The correct sulphuric acid concentration is about 15% by weight. I don't remember the standard concentration of "battery acid" as your starting point; but it's nowhere near 100%. So 10 percent battery acid would be way way low; probably 50/50 is more like it.

Plating/Anodizing Power Supply
0-30V 0-10A

2). I was thinking of getting into paint balling myself. I have an old .22 caliber pistol and I also have one of those ping pong ball toy shotguns where you yank the stock towards you and then the air pressure blows the ping pong ball out. Which do you think would be the better one to use for introductory paint balling?
-- by which I'm trying to make the point that although a battery charger may be your better choice because you need to start with low voltage, neither a battery charger nor a HAM radio power supply is really a very decent anodizing rectifier :-)

3). The positive (anode) lead connects to the work, so by 'paddle' you must mean the cathode (negative lead)? I would suggest a 'flat piece of aluminum' as your cathode rather than aluminum foil I think. It needs to be able to carry the current.

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"


A. Hello Wayne,

I'll try to keep this as basic as possible for you. First of all, you'll want to make sure that your piece is free of any coatings, and free of all screws, and other non-aluminum hardware, and that the piece is as clean as possible. As far as your concentrations of acids go, if you are just doing this on your gun for decorative reasons, you'll want about 15%-20% solution of sulfuric acid to water (the cleaner the water, the better...tap water isn't the best). REMEMBER: ADD THE ACID TO THE WATER...NEVER THE OPPOSITE.

You'll want the bath temp to be around 70 °F. Your Ham radio power supply would be the better of the two, although it really doesn't meet the minimum requirements. Ideally you'll want to anodize at 12-15 amps per sq. ft of surface area of the piece you are trying to coat (your 12 volts may not be enough to accomplish this).

You'll want to slowly (2 min will do) ramp the power supply, not just jolt your piece with the full current. As far as the cathode (-) goes, a piece of aluminum (ideally 6063 alloy) sized to approximately 1/3 of the surface area of the piece you are trying to coat will suffice, you can just hang this piece over the side of your "tank".

If you follow these parameters, it should take approx. 20 min to build .0005" of anodizing. You didn't mention anything about dyeing your piece, so I'm assuming you are looking for clear anodizing. Your part should come out of the tank with a slight grey color (this depends on the alloy of the piece you are coating). Once you are done coating, the part should be thoroughly rinsed in clean water, then sealed. To accomplish the sealing you can boil some very clean water (once again, not tap water), and immerse your piece in the boiling water for a period of 25-30 min. Remember to use rubber chemical rubber gloves [linked by editor to product info at Amazon] and eye protection when working around the acid bath.

Hope this helped you out! There are a lot more fine details that apply to anodizing than what I've mentioned here. Keep in mind that for probably less than $50, you could have this professionally done.

See what you get when you ask "experts"? You get 2 different opinions! I respectfully disagree with Ted's power supply choice :)

Marc Green
Marc Green
anodizer - Idaho

thumbs up signYour anodizing experience greatly exceeds mine, Marc, and I go along with whichever power supply you think is better. My main point was just that asking whether a battery charger or a ham radio power supply was the better choice for anodizing is equivalent to asking whether a .22 caliber pistol or a toy ping pong ball gun is the better paint balling weapon -- that is, neither is very appropriate.

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

I hear ya Ted!

Marc Green
Marc Green
anodizer - Idaho

September 17, 2013

Q. I have to anodize some 7075 Aluminum receivers. I have an ancient rectifier that has a voltmeter that goes to 15 volts, and an ammeter that goes to 25 amps. It has a single large dial that goes from 0 - 100.
It is all I have and finances are very scarce right now. I'm going to be using a 5 gallon bucket for a tank with sheet lead anode.
Is this going to be a lost cause or will I be able to make this old rectifier work.

Ron Paull
- Port Lions, Alaska

September 23, 2013

A. Hi Ron. I think 3 things are important about an anodizing power supply.
1). That you have sufficient voltage. 15 volts should be enough, although it's borderline for alloys like 7075.
2). That you have sufficient amperage. 12-18 ASF should be enough, so 25 Amps should handle parts up to about 2 square foot in surface area.
3). That you can either control the volts or limit the amps. When you start anodizing, you are working with highly conductive aluminum. When you finish anodizing you are working with a surface that has a thick, insulating anodized film on it. So you must start at low voltage (or limit the current) to keep the parts from burning, but increase the voltage (or maintain the current) as the insulating anodizing film builds up and adds resistance.

If you don't yet have experience with 1xxx aluminum or 6xxx aluminum, you might want to start there rather than starting with quite difficult 7xxx. Presumably this is a hobby, not a small business which should have various hazardous waste management permits.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Anodizing for school science project

April 30, 2015

Q. Here's a video that we made for school about the process; you can also see the finished result against the original

Jose Estremadoyro
- Mexico City mexico

May 2015

thumbs up sign Hi Jose. Great work!! (Readers: please click through to view it and like it on YouTube).

But we with experience out in the world are frightened for your health when we view this. Please help yourself and your lab partners stay young looking and handsome/beautiful by always wearing goggles and rubber gloves when working with chemicals! Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 3, 2015

! Did I hear right?

"A thin layer of lead will form on the aluminium"

Please tell me I heard wrong.

geoff smith
Geoff Smith
Hampshire, England

thumbs up signHi Geoff. You didn't hear wrong. They were confused (at one point at least), thinking the lead cathode plates out onto the aluminum anode. Taking it as a whole, I think they eventually learned otherwise but didn't do another "take" to fix their first "chapter" of filming ... and they did get a lot right.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

How to anodize 7075 using a welder as a power source

December 3, 2015

Q. I am a graduating mechanical engineering student and have been playing with anodizing aluminum. I have recently taken to anodizing aluminum for 2 of my hobbies. I am involved in downhill longboarding and my truck hangars (parts that hold the axles and wheels) are made from 7075 aluminum as I have found. The other hobby that I am involved in is firework manufacturing. The vast majority of firework tools are machined from 6061 aluminum and due to their nature have exceptional benefit from having a type III anodize with the teflon additive. Most of the tools are essentially pistons and cylinders with specialized pistons for different purposes, or one tool in particular is called a star plate. This tool is essentially a plate with a square matrix of pins sticking up with a matching female plate. It is essentially a group of piston and cylinders. The purpose of these devices is to compact damp powder into pellets. The damp powder has a water activated glue in it so it tends to stick to bare aluminum. It doesn't stick to the hard teflon anodize though.

This will be a lengthy post, I apologize in advance, but I figure that a clear description will best suit a proper answer to my questions. Here is my current process

I have been experimenting in anodizing different alloys using a 3 amp/ 30 volt power supply under constant current. I "rack" my components with either aluminum welding rod, or 16 gauge aluminum wire. I dip the part in NaOH drain cleaner for a bit to etch/clean the part. Using a clean gloved hand I remove the part, spray it with a degreaser and rinse it in the tap. I then 'electropolish' the part by 'anodizing' it in the green phosphoric acid solution sold in home depot as 'Phosphoric prep and etch.' I notice that when doing this, the voltage is extremely high and as the electrolyte warms up, voltage drops and the current rises to my desired 3 amps max output. So far my parts have been small scrap pieces of bar or tube with a surface area of 11 square inches or so. I notice that this amount of current won't actually electropolish the aluminum, but gets it silvery and clear. In the lab at school I have used a 10 amp source with pure phosphoric and it mirror polishes my parts, but I do not have access to those at home. Sorry, I digress. My next step is rinsing the part by dunking it multiple times in distilled water. I then anodize the part at 3.16 amps and go with the time spec'd in the 720 calculator for 1 mil of thickness. This is usually 30 mins or so to an hour depending on the piece. The solution is 50/50 battery acid/distilled water at room temperature. I have placed the solution in the freezer overnight and anodized in it and my anodize layer was yellow/olive in color, indicating a possible hard anodize? My containers have just been tupperware that hold about 2 cups up to about a gallon. My cathode has been another piece of scrap aluminum about the same size. After anodizing, I rinse the part in distilled water. I heat my dye bath in the microwave until it starts to steam a bit. Not boil or cavitate, but steam a bit. I drop the part in for a few minutes until its the color I want. I dunk the part in boiling distilled water for a few moments then place it in a strainer and put it over a steaming pot for 20-30 minutes.

This method has worked to anodize, dye, and seal some parts using both Rit dye, and the commercial dye from Kingscote chemicals. Sometimes the color will still fade a bit when handling after its been 'sealed.' Here is where my troubles have begun. Thanks for sticking with me this long. I am forever grateful to whoever is still reading this.

My 7075 parts are a nightmare. It will not electropolish to a perfect mirror on my smaller power source. In fact it will still clean and brighten the part, but now it reveals the grain boundaries in the aluminum. It has somewhat of a 'hunter's camoflage' pattern to it. If I use a 10 amp power source, it will polish to a perfect mirror. I can get the 7075 to take dye, but it will rub completely off even after steaming for 30 minutes. I mean completely rub down to colorless again. The dye comes off in a powdery mess. It is very odd. I believe I read somewhere that 7075 takes more power, so I have begun to attempt to use a TIG welder in stick mode. This gives me the option to control the current that flows through. I am using a nice Miller welder and set the amperage to 10%. This equates to roughly 26 amps or so. My part (longboard truck) has a surface area according to my best guess of about 44 square inches. My problem here is that my racking material keeps getting cut in two right where it enters the solution. I figured this was from using too much current for the aluminum wire or welding rod. Interestingly enough, this happened when using my small power source also. I figured I would need to increase the diameter of the rack material to allow more current. I then bought a 3/8-16 stainless bolt and screwed it into my truck very tightly. I then clamp the positive to the bolt. I went through the whole process and noticed that the steel rod electropolished incredibly well. Absolutely beautiful, but now my aluminum is not doing anything. After further reading, I saw that stainless has low current carrying ability. I then cut threads into a 3/8" rod of aluminum and screwed it in to my truck. Same problem. The aluminum rod is now polishing and getting quite warm, but the truck is not. I do notice however this time, that my truck will instantly be covered in bubbles when I turn on the power, but it will not polish. It seems now that it is being etched, not polished. Even when anodizing, the truck will no longer anodize. It will just get hot where the threads are and only anodize right at the point where the aluminum thread screws into the truck. It is the strangest thing.

I have found that I was still seeing the melting/cutting in the stainless bolt. I find it odd that even with my small power source, my hanging wire would be cut in two when anodizing. I ultimately want to type III anodize all my parts, but want to get familiar with type II first. So now finally to my questions.

After seeing my process, what am I doing incorrectly?
How should I properly rack my parts?
Why is my current 'rack' only polishing, but my part isn't, even after being screwed in tightly.
Why are my Al parts no longer polishing but etching in phosphoric?

This is an exhausting, yet fun challenge. I am very eager to overcome this obstacle.

Jarrod Frankum
Hobbyist/student - Terrell, Texas USA

February 2016

A. Hi Jarrod. First, you can't put steel or stainless steel into the anodizing bath. All the current will flow to it, and no anodizing will occur.

I'm confident that you can buy much heavier aluminum wire, such that it will not heat up or burn through. It should be good for about 600 A / sq. in. But the problem remains that the wire will tend to suck up the electricity that you are trying to direct to your 7075 parts.

A racking approach that I've seen used on architectural extrusions, which might or might not work for your truck hangers is aluminum screws that would go through your trucks and into threaded holes in a permanent contact block. The heads of the screws press against the truck, and the screws carry the current from the tapped holes in your contact block to the heads of the screws and the trucks. The tapped holes tend to not anodize, both because they are tapped holes and because there is a screw filling them during use. You can replace the screws or strip them in the NaOH each cycle.

When all the dye rubs right off, it probably means the surface is not anodized (anodizing makes pores or drill holes that absorb the dye, and you can't rub the dye out of those pores). But failure to properly seal will mean the dye rubs off too easily too.

If you've been able to successfully anodize with the right equipment in school, you're doing pretty good! ... and should probably focus on how you can use the right equipment rather than making other adjustments to try to compensate for using inadequate equipment :-)

Luck and Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Question about amp draw on my anodizing tank

February 12, 2016

Q. I've been anodizing RC car parts for about 4 years now and have pretty good success. Recently I've noticed that to achieve some colors I've had to anodize for longer periods. As of now for 7075 my anodizing time is 60 minutes and for 6061 it's 80 minutes. Now I didn't think much about it until talking to a fellow anodizing shop close by and he told me that his 6061 time is 30 minutes and 7075 obviously less. So that got me thinking: where am I losing voltage, amps or whatever I'm losing that's costing me 30 minutes per run.
Our setups are nearly identical, we both use constant voltage @ 15v, both have 10 gallon tanks, both use titanium racks bought from same store, both use same acid ratio of 16% by weight. The only difference that I can tell is that I use an aluminum grid anode that I bolt my part racks to and his anode is a copper bar that he hangs his racks on.

Just before typing this I checked the voltage going through my anode and with rectifier set on 15 volts I was only getting 13.2 volts from cathode to anode. To get 15v through the grid I had to raise voltage to 17.1. With the lambda power supply if just check leads I get 14.8 with digital readout set on 15.0, So does that mean I'm losing the 2 volts in my grid, and if so would that double my times?

Sorry for long winded post just trying to be thorough. Thanks in advance.

Jason Bowlin
Small shop - Hays, North Carolina USA

February 2016

A. Hi Jason. If you search this site for "720 Rule" you'll learn that the thickness should be directly proportional to the current, such that if you are losing 15% of your current you should be losing 15% of your speed, not 50% of it. Additionally, I don't understand your issue of 6061 taking 33% more anodizing time as 7075; it should actually take slightly less time.

Do you have any idea what anodizing thickness you are shooting for? Maybe 0.0002" is enough for lighter colors, but 0.0003-0.0004" is needed for darker colors? Probably even a little more would be required for a robust black.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

February 15, 2016

Q. Ted, when I was researching this several years ago, I thought I read several times that under the same conditions that 7075 and 2000 series aluminum took less time at same voltage to reach same anodic layer. I have since been told that by my tech guy at the chemical supplier I use.

As far as the size of the layer, I haven't really been that technical as long as I was able to get the desired color. I have noticed that black does require more time.

The times I'm using now produces great colors; I was just hoping to figure out why my times are so much longer than industry standard.

Jason Bowlin [returning]
- Hays, North Carolina USA

February 2016

A. Hi Jason. I'm not a hands-on anodizer, but I've never heard of that claim about 7075 anodizing twice as fast.

The 720 Rule will tell you what the "industry standard time" is to get to any given thickness, but it's impossible to say what the "industry standard time" is to get to an unknown thickness. The case may be that your competitor is doing better dyeing than you, and thus doesn't need as much anodizing thickness to get to the saturation level that you are looking for.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

simultaneous February 19, 2016

thumbs up signThanks for the response, after reconfiguring my line with copper bus bar and all titanium hangers on my racks my times have dropped drastically. I was able to run full load of 6061 last night for 40 minutes @ 15v and achieve the desired color.

The problem with my type of anodizing and the 720 rule is I do mostly stripping and re-anodizing of tiny parts that are all different sizes and measuring each piece for each job would be a full time job, so by trial and error with my setup I have to learn what time at 15v gives me the color I'm trying to achieve, whatever thickness that is.

Jason Bowlin [returning]
- Hays, North Carolina USA

February 20, 2016

A. Ted, I think the confusion here is that you're talking about the 720 rule (using constant current), while Jason is anodizing by constant voltage. I could easily see the 2000, and 7000 series aluminum anodizing quicker using constant voltage, hence the reason anodizers prefer to use constant current, as it's a repeatable and much more predictable process when anodizing to a desired coating thickness.

Jason, use constant current if possible, not voltage, and I think you'll find the answer to your question.

Marc Green
Marc Green
anodizer - Idaho

May 8, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi all new to this but done a bit of research and come to a dead end there's nothing on what I want to achieve regarding anodising --

I want to anodise and dye dirt bike rims and hubs and nipples
I know the consequences regarding cast hubs but what I do want to know are the following

Can any sheet lead be used as a cathode or is it specialist (I have a roll of 1 mm roofing lead lying around)

Will any car battery do (e.g., 12 V 45 Amp)? Do I need a regulator or anything else?

Would appreciate any advise thanks.

Tony Daton
Hobby - England

May 16, 2016

A. 12 V is not high enough. You need 15 - 18 V.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

May 21, 2016

Q. Sorry I have 2x 12 V batteries, so 24 V

Another question: I have mixed 4 liters of 32% battery acid to 20 liters distilled water is this correct?

Cheers, tony

Tony daton
- United kingdom

A. Hi Tony. It's much too weak. You are looking for 15% by weight, which means that a 1:1 mix of 32% battery acid and distilled water would be quite close.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

June 3, 2016

Q. Where does all of the conflicting information end? Over the past week or two I think I have read and watched every YouTube video on the subject of Anodizing Aluminum. I have seen everything from a Redneck Brit with a 5 gallon bucket and aluminum foil with a battery charger to some guy talking about the finish on an Ipod. I have tried and failed at coloring Anodized Aluminum of the 6061 and 7075 variety.
I tried the battery charger method using a unit capable of producing up to 60 amps. I'm not sure if the equipment is the issue or not, so I just ordered a bench top power supply from Ebay that is adjustable from 0-30 Volts and 0-10 Amps. I just need someone to tell me how to use it.
Here is what I have; in a tub I have mixed 6 Quarts of Sulfuric Acid from an Auto parts store with 1 gallon of distilled water (water first, not the other way around). 2 aluminum sheets 1/4 inch thick 11 inches long and 6 inches wide, hooked to the negative lead. 1 bubble agitator and a cooling system to keep the acid between 70-75 °F. 1 exhaust hood to ventilate fumes outside. Titanium rod threaded into my work piece.
If I have a piece of 6061 that is 3 inches in circumference and 6 inches long (capped off) and I only want to "color" the outside, this should be about 18 square inches. How long should it be left in the tank and at what volts / amps do I set my power supply to? Also, the same questions with a piece that is 60 square inches of 6061.

Thank you in advance for the information.

David Dunbar
- Eskridge, Kansas, USA

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon

June 2016

A. Hi David. Nothing wrong with youtube videos, but I see conflicting youtube info on the simplest stuff like replacing washers on shower faucets, whereas anodizing is an industrial science with 1500-page textbooks, at least two trade associations / educational associations, annual conferences, and people who have devoted their entire careers to it. Obviously it's tough to condense such a subject into a quick video or a couple of paragraphs of text.

Nevertheless, a good amperage for standard anodizing (as opposed to low current density hobby anodizing) is about 12-18 ASF -- which means 2.25 Amps for your 18 square inch part and 7.5 Amps for your 60 square inch part. At the beginning of the cycle it will take little voltage to generate the 18 ASF; as the anodizing builds and the conductivity of the surface drops, the voltage will climb to 12-15 Volts if you hold 18 ASF.

The 720 Rule will tell you how long to anodize for depending on the thickness you want. It takes 720 Amp-minutes / square foot (or 90 Amp-minutes for an 18 square inch part) to build 0.001" thick anodizing. If you anodize at 12 ASF, that's 60 minutes; at 18 ASF, that's 40 minutes. But 0.001" is pretty thick ... you probably want half of that.

Your acid is probably okay for casual use. How do you know you are not getting any anodizing, or that the material isn't already anodized? And what is happening? What do you see? How much current is flowing? Do you see gas bubbles on the anode and cathodes?


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

June 5, 2016

A. Ted's advice is sound, David. One of the things he stated is very important, and that is are you sure the part isn't already anodized (some very thin coatings can be hard to see). A simple continuity test with an ohmmeter can confirm. Touch one lead to your Ti rod, and the other to your part, and see if you have continuity. If not, your piece is coated with something, and that coating needs to come off. You can also use a continuity test to see if you're being successful in your attempts to anodize.

Your power supply sounds adequate for the parts you're trying to coat. If you coat per Ted's advice, in approx 20-25 minutes, you'll have grown approx .0005" of coating, which should be fine for what you're trying to accomplish. Set you're voltage at 30, and your amperage at 0. Turn the unit on, and slowly (around 2 minutes) increase your amperage to the appropriate amount for the work you're trying to coat, in this case, 7.5 amps.

The other thing I noted was your acid concentration. You should be around 15%-20% of acid to water. The way you described your set up, (1 gal of water to 1.5 gal of concentrated acid)is way too strong.

You also mentioned "coloring". That's another step done after the initial oxide is grown. I think home anodizers have had success with RIT dye. And finally, there is a sealing step done, which can be accomplished in your home with boiling water (distilled).

Marc Green Marc Green
anodizer - Idaho

June 8, 2016

Q. I now have everything ready for the tubs to the electrolyte mix.

Coated my tub in lead.
2 x 45 Amp 12 V batteries.

I'm going to try and oxidise an 18 inch motorcycle rim

probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert

Are there any books out there that anyone can recommend? Im after anodising and dying items of this size.

Cheers tony

Tony daton [returning]
- United kingdom

Ed. note: We have a vested interest, but we suggest Robert Probert's "Aluminum How-To" =>

DIY Anodizing problems

March 10, 2018

Q. Hi all!
First of all this is a wonderful site and community and has been a tremendous help already.
I just have some more specific problems that I haven't been able to find answers to.

I am trying to colour anodize a set of wheel spacers, about 200 mm in diameter and 25 mm thickness.
The alloy doesn't seem to be the best for anodizing, as when I put the spacers in the NaOH solution, they come out almost completely black. To remove this, I dip them in 58% HNO3. This removes the blackness instantaneously, but then some pitting or uneven patchiness can be seen.


I then have made an anodizing bath where I've made a solution of NaHSO4 (the pH is around 2), I have a polished stainless steel flat bar running across the bath and as a cathode I've bent a sheet of aluminum into an L-shape, which covers about 75% of one side and the the bottom of the tank.
I then used aluminum mig-welding wire to tie the spacers and hang them from the stainless bar so that they are completely submerged in a vertical position.


I used an old truck battery charger running at 24V, positive clamped to the stainless bar and negative to the L-shaped sheet of aluminum. When I switched it on, a lot of fizzing ensued and the ammmeter on the charger showed a current of about 20-23A.
The problems then were:
-the wires, which suspended the parts, kept snapping, regardless of how many strands I made (started with single wire and by the end folded it 3 times to get 8 strands)
-when I finally took the parts out and rinsed them, the were uneven, slightly rainbow-coloured and they didn't really want to take in any dye, only very barely.

What should I be doing differently?
I've read from several sources that the cathode shouldn't be on the bottom, but why? Is this my issue?

I want to redo the parts, so I am thinking of removing the anodizing in the NaOH solution, then sanding and buffing them, then clean and then retry the anodizing.

Thank you!

Riido Kolosov
Hobbyist, Mechanical Engineer - Tallinn, Estonia

March 2018

A. Hi Riido. Starting from the top of your posting --

Professionals use inhibited chemistry for cleaning, not straight NaOH. Try cleaning the parts by scrubbing with powdered pumice and rinsing first, then dipping into your NaOH for just a couple of seconds, using it strictly as a minimal etch, not a cleaner.

I'm personally not familiar with anodizing in sodium bisulphate and would suggest 10% by volume / 15% by weight sulfuric acid instead. You can probably add battery acid about 1 part to 1 part water for this.

I'd try one part at a time instead of all four until you've learned a bit more. I figure they'll draw about 10 Amps each.

Ideally you should have anodes on both sides rather than one side and the bottom, but I don't think that's one of your major problems.

24V is way too much starting voltage; or you need to limited amps; a battery charger isn't very appropriate for anodizing.

Your aluminum mig welding wire isn't cutting it for carrying the amperage. Maybe take some of that excess aluminum anode sheet and try to "wire" up one spacer with it.

The rainbow coloration probably indicates that you have an anodizing layer of partial wavelength thickness, probably about 1/10 to 1/20 of what you need. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

March 13, 2018

Q. Hi, Ted!

Thank you!

What would the appropriate voltage be? The other option for me would be a car battery charger, which charges at around 14.7V.

Is there any good tip on how to attach the anode wire/aluminum strip to the spacer?

Best regards,


Riido Kolosov [returning]
- Tallinn, Estonia

March 2018

A. Hi again. Battery chargers are not great power sources for plating or anodizing. One problem is that raw aluminum is highly conductive (you're even using aluminum wire to carry power to your spacers) but anodized aluminum is highly insulating / very high electrical resistance. Since A = V/R, too much current can flow in the beginning, causing burning, and too little in the end, so the coating never gets thick enough. Ideally you either start with low voltage and ramp it up slowly, or you anodize at constant current. But if you determine the maximum current the charger can put out, and compare it to the 10 Amps per spacer which I estimated, maybe you can get by.

Whether you can practically bend your anode material to make a good connection, I don't really know. But what you could do is go to an electrical store or building supply store and buy the right kind and gauge of aluminum wire. In the USA aluminum wiring is used on service entrances to circuit breaker boxes (but not in house wiring). If it's the same in Estonia, maybe you can find 10-gauge aluminum wire instead of your MIG welding wire. Of course, 8-ga or 6-ga could be used if you can't find 10-gauge. I don't know how you'll avoid "rack marks" where the wire attaches to the spacer, because you need a solid, fixed, connection because otherwise the wire and the spacer will anodize at the connection point and you'll lose contact.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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