Letter 1318

How to paint aluminum

--- We are evaluating the possibility of installing a new powder paint line for aluminum extrusions and are having some doubts about the proper preteatment for paint. Can anyone help? Guillermo Luna - Mexico City, Mexico --- Proper pretreatment for aluminum castings and extrusions takes place in at least a 5 stage pretreatment system. The system would consist of the following stages: stage 1: Alkaline cleaner (aluminum-safe, or mild etch) Stage 2: Fresh Water rinse Stage 3: Iron Phosphate solution containing fluoride Stage 4: Fresh Water Rinse (Recirc DI Optional) Stage 5: Fresh Water Rinse (Virgin DI optional) Choice of cleaner depends on alloy and soils to be removed. The fluoride is the key. It etches the aluminum surface to provide for good paint adhesion. Reduce the stages and you reduce the chances for success. Craig Burkart - Naperville, Illinois

Guillermo, the suggestion Craig Burkart gave you is a very good suggestion. Remember that aluminum does not accept a conversion coating. You must clean the metal and etch it for best adhesion. The D.I. water will help you to rinse any residual contamination from the substrate prior to the next step I.E. ecoat/wet spray/powdercoat etc. By using a D.I. virginal rinse, the substrate is left in a slightly acidic state the powder just loves to stick to. You could also use reverse osmosis water if you like. The other area of interest to you may be in making sure the grade a casting/extrusion is compatible with the cleaning chemicals to prevent increase cleaning demands or multi chemicals to alleviate problems such as smutting. Bob

Bob Utech Brooklyn Park, MN

Editor's note:  Mr. Utech is the author of -->

A Guide to High Performance Powder Coating [link is to Amazon]

Bob, aluminum does accept chromate conversion coating. See Mil-C-5541 [link is to spec at TechStreet]. When chromating prior to painting the salt spray resistance will be far better.

I assume that there are some applications that phosphate will do instead of chromate.

Sara Michaeli     chemical process supplier Israel

First and foremost: How much corrosion protection do you need?

If your requirements are minimal, then iron phosphate is fine. If your requirements are extensive, then here's what I do. It's not perfect, but it works.

1. Alkaline Clean, inhibited and chelate free, 90 seconds.
2. Recirculated rinse, with a fresh water make-up riser, 60 seconds.
3. Chrome Phosphate, 30 seconds.
4. Recirculated rinse, with a fresh water make-up riser, 60 seconds.

We apply the Chrome Phosphate to a coating weight of 50 - 125 milligrams/square foot.

The parts are then powder painted with a TGIC Polyester at 2 mils (50 microns).

We experience over 3000 hours of salt spray when tested with a stainless steel galvanic corrosion cell attached.

Some things that I would like to do if I could.

1. DEOXIDIZER: Extrusions, I usually see 6061-T6, tend to have a heavy oxide film, that needs to be removed either chemically, acid deoxidizer, or blast clean.
2. RINSE: I want lots of rinse tanks that back flow, thus significantly reducing water usage and providing cleaner rinses.
3. NON-CHROME: This is emerging technology. Some testing I've done or seen results indicates that there is some really good stuff.

If you use chrome phosphate, or chromium chromate, ALWAYS install a heavy metal pretreatment system.

Kelly Loch

Sara, yes of course, aluminum accepts a chromate conversion coating. I think Bob was using the lingo of painters and pretreaters whereby 'a conversion coating' is shorthand for 'an iron or zinc phosphate conversion coating'.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

According to me, the best pretreatment for aluminum prior to powder coating to ensure best adhesion and prevent filiform corrosion consists of:

1. Alkaline degreasing
2. Rinse
3. Caustic etch
4. Rinse
5. Nitric acid desmut
6. Rinse
7. Yellow chromate
8. Rinse
9. Rinse in Deionized water
10. Dry at max 70 deg.Celcius

There are several Cr-free products approved by the European organization QUALICOAT to replace "yellow chromating" if wanted.

Aluminum has a natural oxide film on surface and depending the time between production process and surface finishing, this film may reach substantial thickness. The pretreatment above is advised to get the best results after powder coating.

You can follow the specs given by QUALICOAT of Europe for powder coating.

Timur Ulucak

We have several extrusion lines running a Chromium-free alternative equivalent to the extrusion industry standard Chromium phosphate. AAMA 603 [link is to spec at TechStreet] and AAMA 605 [link is to spec at TechStreet] call out most of the proper specs. for this industry.

Shawn Dolan Henkel Technologies Madison Heights, MI

Dear Guillermo Luna

Pretreating and painting of aluminium extrusions can be a complex issue and you will find much, sometimes conflicting, advice around.

Installing a new plant, you have the opportunity to learn from others mistakes and take full advantage of new techniques and ideas.

I have been managing plants anodising, painting and powder coating aluminium extrusions for over 15 years, and currently manage 2 plants coating 3000 tons of extrusion a year.

Ian Peart
General Manager (Finishing) - U.K.

What is the fastest and most economical way to remove a large quantity of latex (gag!) paint from aluminum? Someone recommended "aluminum aircraft paint stripper." Okay, Wal-mart doesn't carry this. Any idea who does? Thanks.

Chad Krus
- Overland Park, KS

Ed. note: You can get Aircraft Stripper [link is to product info at Amazon] on the net, Chad.

First, let me qualify by saying that I'm an amateur in finishing. My work does involve painting steel and aluminum in the marine environment, but only periperially - I don't spec that out. I have been vigorously researching aluminum finishing for amateurs, in conjunction with my interest in homebuilt airplanes.

The process I suggest for most amateurs runs like this:

1. Alkaline degrease

2. Rinse

3. Mechanically remove oxidation

4. Rinse

5. Self-etch primer, applied as soon as possible after the rinse.

6. Top Coat

The degreaser can be anything, well, alkaline and degreasing. Dawn brand dishwashing detergent is fine. Your local auto parts store has something.

Rinses should be hot, if at all possible.

Maroon Scotchbrite pads are the best way to remove oxidation, either the hand pads, or the drill motor mounted "Surface Conditioning" pads. Degreasing and oxide removal can be combined.

Most auto parts stores can at least order self-etching single part non-chromate primers in spray cans or quarts for brushing with a foam brush (problematic, holidays are badddd), roller-brushing (roll an even coat on and tip it with a foam brush, better) dipping or spraying. NAPA carries Martin Senour 7220. Marhyde makes another one. These are one part alkyd systems that have very low health risks, a simple particle respirator is semi-adequate protection. It's crucial to apply the primer as soon as the surface is dry from the final rinse. A mild solvent wipe (alcohol) can be used to speed up drying.

Top coats with this simple system can be any single part paint, from latex to moisture cure epoxies. Acylics with strong solvents, and any two part polyurethane will lift this primer, and ruin the job. But amateurs should not be using polyurethanes, and especially not spraying them. The isocyanurates will kill them without a supplied air respirator.

This is a "good enough" system. It will NOT produce the same level of performance you'd get by using more toxic forms of degreasing (MEK anybody?), acid etching/deoxidizing, conversion coating, chromate containing primers, and more expensive, nastier paint systems. But it reduces, substantially, the health risks and the toxic/hazardous wastes that consumers DO NOT carefully dispose of.

Aviation Paint Remover [link is to product info at Amazon], the active ingredient is the same stuff, Methyl Chloride, as any useful (as opposed to useless) paint stripper. It's safe to use on aluminum.

I heartily welcome feedback on this system, as I plan on promoting it's use to homebuilt airplane folks. I see too many articles in the aviation hobbyist press advocating the use of "industry standard" practices of 40 years ago. The aircraft industry has done a tremendous job in the last 10 years of reducing it's hazards and it's waste stream, it's time the the hobbyists did the same.

Ryan R. Young
- Oakland, CA, USA

I have some brand new clean- bare square aluminum tubing, 3/4" and 1" sizes, and I would like to paint or color coat for appearance. It will used outdoors.
Does anyone know of any color coating that might be easily applied and might last a few years?
Thanks, JIM

Jim L. Russell
developer - Tulsa, OK USA

I want to repaint my store front window frame--it came painted smooth green. I am presuming it's anodized and need to know if I need to sand and prime it before painting it. Some paint has flaked off. Also, what it the best paint to use for this surface (I'm also painting a non-painted aluminum door) for this job. Can I find it at ACE? I was going to use a spray can for a smoother finish since this is will be exposed at eye level.
Thank you!

Kelly Corral
Consumer of products. - Chicago, IL

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