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topic 14122

Re-anodizing Aluminum


A discussion started in 2002 but continuing through 2019

2002

Q. Can you re-anodize a piece of aluminum once it has been anodized already? I have a piece with some heavy scratches and scrapes and want to try to make it look a little better. Any suggestions?

Jim Stokes
- Richmond, Virginia


probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert
$89

2002

A. It's a very standard practice to strip and re-anodize, Jim, and very often it's no problem.

The only issue is that about a thousandth of an inch of aluminum is consumed in building a two thousandth of an inch anodized coating. When you strip that coating, the aluminum is lost it doesn't come back. So if the dimensions are critical you can have a problem there.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



May 10, 2010

Q. Do I have to strip the first anodized layer to re-anodize the part? Here's my situation. I'm making some pistol grips for a 1911. I have the grips to the shape that I want them. I plan on anodizing and dyeing the grips. then I will use a small diameter drill bit and drill lots of shallow "pits" in a honeycomb pattern. I know that once I drill into the anodized surface, those drill areas will not be anodized anymore. I want to leave them the natural shiny aluminum color, but how do I anodize the "pits"?

Patrick martin
- st. marys, Georgia


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

May 11, 2010

A. Hi, Patrick. I don't believe there is any technical reason that you can't do this, but there can be some problems with technique. Anodizing is not resistant to either acids or alkalis, so it will be a problem to clean those drilled pits.

Also, if you do not want to go to the cost and effort of masking the dyed anodized surface, your second anodize may have some aesthetic effect on the first layer. One layer of anodizing is used as masking for a second layer sometimes, so I'd say it's do-able but that you want to practice your technique on scrap, not on precious parts. Probably better to mask the anodized grip, and drill through the masking.

Good luck!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



November 9, 2010

Q. A 1950's vintage bus with gold anodized stainless steel exterior panels is beginning to show its age. I don't know much about anodizing- what would you recommend here? The panels cannot be removed, but the anodizing is worn away in some areas...

Pat Robinson
- noblesville, Indiana


November 10, 2010

A. Hi, Pat.

First, wash the panels and look at them wet. If they look good enough wet, it may be possible to apply a clearcoat like Everbrite [a finishing.com supporting advertiser] to maintain that wet look. There are "brush anodizing" services you can retain to do such a project in place, but you may find them prohibitively expensive. An option would be paint that is designed to imitate anodizing. It doesn't really look the same, but may be good enough for a 50 year old bus if you are talking general serviceability as opposed to appearances in car shows. Letter 26417 "True gold anodizing spray paint?" shows real gold anodizing vs. paint that is designed to imitate it. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Re-anodizing the laser etched areas of anodized parts

February 27, 2015

Q. I have access to a laser cutter and have used it to take off anodization on many things to produce designs and whatnot. I was wondering if I lasered an anodized part could I reanodize the part so that only the newly exposed metal would change color, and the anodization that is still on the metal stay there.

Sullivan Evans
student - Troy, Michigan, USA


March 5, 2015

Sullivan

I've never tried it on a laser cut surface; only on machined surfaces. I would expect that the 'bare' surface exposed by laser would have a relatively thick oxide film present. Your challenge would be how to remove it and not the existing anodize.

Willie Alexander
- Colorado Springs, Colorado



Re-anodizing 60 year old aluminum

March 6, 2016

Q. I am restoring a 1952 International Harvester Refrigerator. The door shelves are aluminum anodized with a gold color. The shelves are then trimmed out with white paint.

The original gold color surface has taken a beating over the years, and the shelves need re-anodizing. I took the pieces to a local Anodizing Company. They told me that the older style aluminum is a different composite than today's aluminum, and that the anodizing process used today (chemicals, etc) is different than what was done "back in the day". They said that there could be a possibility that the resulting gold color could turn out "blotchy". They also weren't keen on painting the white paint over the anodizing because it may not adhere well -- especially if I wanted to follow up with a clear coat paint.

They suggested powder coating as an option instead. Thoughts?

Gary Costel
- Butler, Pennsylvania, USA


March 2016

A. Hi, Gary. I think the anodizing shop gave you an honest but slightly misinterpreted answer. I don't think it's a question so much of how aluminum was made 60 years ago vs. how it is made today, but rather the fact that a 60 year old aluminum alloy may by now have separated into its constituents along grain lines such that nice anodizing could be problematic.

Powder coating sounds to me like a practical answer. Good luck.

Regards,

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



January 25, 2019

Q. I have a piece of aluminum 6063 which has been anodized once. I need to machine off some surfaces and perform second anodize. My question is: do I need to strip off / reactivate the first anodic layer on the unmachined surfaces? Or just a neutral pH detergent to remove CNC coolant and then perform second anodize?

Roger Tan
- Singapore


January 2019

A. Hi Roger. Best to strip off all of the anodizing, then do the machining, then clean & re-anodize. It's possible to leave the existing anodizing in place and re-anodize the newly exposed aluminum but on onesy-twosy work, with no opportunity for development and testing, it's probably not a safe idea.

Regards,

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



sidebar2 February 22, 2019

Q. I have a house with large amount of aluminum siding in a purple color. Over time the color has faded unevenly and I want to re-anodize the Aluminium. What process do I follow here? Does it need to be sandblasted first to remove the first layer and then get it anodized? Or would the best option be powder coating it? Trying to consider cost here too since right now I'm completely unaware what it may cost.

Marina Shimanski
- Washington D.C.


adv.  everbrite banner

February 2019

A. Hi Marina. I strongly doubt that the aluminum siding on a residence was anodized. The siding is probably painted.

Look at it when it's wet; if being wet restores the finish sufficiently for you, you can simply have it clearcoated. If it still looks faded when wet, you can have it painted by a house painter.

Regards,

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



March 26, 2019

Q. Hello, my name is Alex. I have two anodized aluminum pistol frames - one from the late 1905's and one from the early 1960's that have some wear. Since tolerances are important to the internal dimensions, is there a way to anodize the frames but not remove the original anodizing so that only the bare surfaces get "touched up"? Or maybe apply some coating that will prevent the internal surfaces from being anodized? Do you have better advice?

Also, if I remove the anodizing (and therefore, a couple of thousandths of material) and hard anodize, will that build it back up to approx. where it is now?

Anodizing is much more technical than I previously understood!

Thank you.

Alex Van Gorden
- Wallingford, Pennsylvania, USA


March 2019

A. Hi Alex. As you say, anodizing is quite technical, so the idea of re-anodizing bare or thin spots without removing the existing anodizing sounds very doubtful. But it may be possible to mask the interior, then strip the coating on the outside and re-anodize it, leaving the interior unaffected as you re-do the outside.

About 0.0001" of aluminum is consumed in building an anodized coating of 0.0002". So, on a diameter (because you are measuring coating thickness twice), after anodizing to 0.0002" thick the diameter will be about 0.0002" larger than you started with. If you started with a diameter of 1", then after anodizing to 0.0002" thick, the diameter will be about 1.0002". If you strip the anodizing, the diameter will be down to 0.9998". If you were to anodize the second time to a thickness of 0.0004", the diameter after anodizing would be 1.0002" again. So it might be theoretically possible to restore the original diameter by anodizing twice as thick the second time around. However, there are three problems:
- first, the buildup is approx. 50%, not exactly;
- second, this would be a job for a high skilled anodizer, not an experimenter; - third, hard anodizing is usually about 0.002" thick, so if you strip it and lose 0.004" from the diameter, you'd have to re-anodize to a thickness of 0.004" to secure the buildup, and that's probably not practical.

So masking the interior sounds like the most practical approach.

Regards,

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


March 29, 2019

Q. Hi, my name is Caleb. I work for a historic restoration company that is currently on a project restoring an anodized aluminum storefront built in the 30's.
The storefront itself is pretty straight forward. gentle detergent and soft scrubbing seemed to work well. however, there are also ornate cast "screens" that attach to the storefront. These, along with more exposed portions of the storefront, are pretty severely pitted and discolored. I believe these areas have already lost their coating? I'm also concerned that harder scrubbing or sanding may remove, or may have already removed, some of the anodized finish.

14122-1a   14122-1aThm   14122-1b   14122-1bThm

I'm hoping to find the quickest/least expensive solution to cleaning these. Just trying to achieve a cleaner more uniform appearance and address the worst of the pitting. Is it necessary to re-anodize the entire surface? If we just scrubbed and didn't re-anodize anything, what kind of corrosion might I be looking at in the future? Could you clearcoat to help protect the areas that have lost their coating? or would it just be faster and better quality to clean and then re-anodize with some sort of paint on product? any advice is appreciated as aluminum is not something we are used to working on...

Thanks so much,
Caleb

Caleb Joy
historic restoration - Seattle, Washington USA


March 2019

A. Hi Caleb. Quite a beautiful storefront!

Detergent should not hurt anodizing as long as the detergent is not strongly alkaline. And scrubbing should have no effect (it's reasonably hard to remove anodizing with sandpaper).

Re-anodizing pitted material is troublesome, so I'd probably suggest the clearcoating.

adv.
Our supporting advertiser, Everbrite [a finishing.com supporting advertiser] has been involved in significant restoration works per my understanding, and contacting them would probably be worthwhile.

Regards,

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



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