How to Paint Aluminum -- and Keep It Painted

Aluminum is notoriously hard to paint. Well, it paints easy enough, but the paint doesn't stick well and easily scratches off or even flakes off later. After trying various formulas, recipes, and magic incantations, none of which worked very well, I came across this article that was published in 73 Magazine back in 1981 (full magazine available here.) Based on that article, I developed a modified procedure that has worked very well for me. This Instructable covers my procedure. It is more involved than just grabbing a can of spray paint and spraying, but it isn't hard and doesn't take very long. It does involve some unpleasant chemicals, but they aren't too bad. The results are worth the effort.

Why is it so hard to get paint to stick to aluminum? I'm no chemist. I took chemistry early in college and tried to block it out after that. So, take everything I say here with a few million grains of salt. But my understanding is that most paints don't bond well with the aluminum itself. In addition, aluminum oxidizes very rapidly, creating aluminum oxide on the surface. Apparently aluminum oxide is even harder to get paint to bond to. It is also very hard; a lot of sandpaper and grinding stones are made of aluminum oxide. How fast does it oxidize? "Very fast" isn't a very scientific answer, but it is the only one I have found. Well, some people say "instantly" or "almost instantly." Not any better. After much searching, I still don't have any numbers.

If the paint won't bond to the raw aluminum or aluminum oxide very well, what can we do about it? First, as with painting anything, it needs to be clean. Very clean. That helps. Second, we should remove as much of the aluminum oxide as we can. Third, chemically change the aluminum to something the paint will bond to better. And finally, use a paint that is designed for the purpose.

Here are the items I use and recommend.

• Liquid dish soap. The liquid dish soap of your choice that cuts grease well.
• Self etching primer. I have mostly used the Rustoleum brand, but I have also used some bought from Harbor Freight that seems to work just as well. The key ingredient seems to be zinc phosphate. Original formulas used zinc chromate and it may work better if you can find it, but zinc chromate is REALLY NASTY stuff. I will stick with zinc phosphate, thank you.
• Phosphoric acid. Commonly sold as etch prep for concrete and rust remover/converter for steel. It's a rather mild acid (it's actually added to soft drinks to give them a tart taste.) The naval jelly mentioned in the original article is a jelly containing phosphoric acid.
• 320 grit sandpaper.
• Scouring pad or scouring sponge.
• Two or three containers to soak the aluminum part for cleaning, etching, and rinsing. You can reuse one for both cleaning and rinsing, but use one for etching only. The acid bath leaves a nasty residue.
• Paper towels.
• Rubber or latex gloves.
• Other solvents and cleaning agents as needed for cleaning the aluminum.

The first thing we need to do is get the piece really good and clean. Any type of oil on the aluminum will keep the paint from adhering. Oil from your skin is very easy to get on and invisible -- until the paint goes on.

One thing to note is that sheet aluminum is often sold with a clear, thin plastic stuck on one or both sides. It's a bit hard to see. Make sure you remove it if it is there. See the photo.

If there is any crud stuck on your piece, remove it by scraping or cleaning with some type of solvent.

Now fill a container about half or three quarters full with clean, hot water and add the liquid detergent. Squirt some directly on the workpiece, too. Using your scouring pad scrub it really well, making sure you get every corner and crevice. Dip it in the soapy water a time or two. Scrub it while it is in the water. I usually spend about five minutes scrubbing, then put the piece into the soapy water and let it sit while I prepare the next steps. I don't time it with a clock. I just sing one or two of my favorite songs to myself. My singing is bad enough that I can't manage more than that.

Now, put on your gloves so that you don't get skin oil on your freshly cleaned part. Take it out of the soapy water and scrub it under warm or hot running water to clean all the soap off of it. Keep your gloves on from now until you have the part covered with paint.

Using 320 grit sandpaper sand the part thoroughly, much like you scrubbed it clean. Make sure to get every corner. Inside corners are tough and I find emery boards and such help. Use whatever works. Make sure you get the whole surface and scuff it up good. This helps prepare the surface to hold the paint and helps remove any remaining dirt or soap. It may also help remove a bit of oxidized aluminum.

Waterproof (wet or dry) sandpaper and wet sanding is good, but dry sanding works well, too.

The first photo shows the part half-sanded. It's hard to see in the photo, but the sanded part on the right has a different sheen than the unsanded part on the left. In real life, it is much easier to see. You want a nice, uniform, sanded sheen over the whole part.

You can follow up with finer grit paper if you want a smoother surface. I rather like a matte look and the primer tends to smooth it out anyway. The rougher surface may hold the paint better, but if you want it smoother sand until you are happy. My wife tells me I'm a bit rough, and that's how we want our aluminum.

Now rinse off all the sanding dust, wipe it with a clean paper towel, rinse again, and wipe it off again. Now it is ready for the next step.

Fill one container about half to three quarters with the phosphoric acid. It should be deep enough that the part can soak easily but not so deep you have to stick your whole arm in to retrieve the part.

Drop the freshly sanded part into the acid. Try not to get any acid on you. It is fairly mild, but it will burn you. Don't ask me how I know this.

Let the part soak for 30 minutes or so. It's a good idea to flip it over a time or two while it is soaking to make sure the liquid can get to all the nooks and crannies. I go find something else to do; I can't stand 30 minutes of my singing. Maybe read the news. 30 minutes of world problems is about all I can take. The time isn't critical at all but I do find around 30 minutes works well.

The photo shows my well-used container of phosphoric acid. Note the color: it changes as it is used. I suppose it is from the chemical reactions. But it still works well. Also note the crud on the sides of the container. That is interesting stuff. It is really hard and really hard to clean out. That's one reason I don't recommend using this container for anything else, especially not rinsing. But also, I just keep this container filled with phosphoric acid for various uses. It's also really good for rust removal, but that's a different story. The coffee cup was a gift from my rocket-scientist daughter when she was in school. A bit of gratuitous bragging never hurts! :-)

The second photo shows the reaction taking place in the goop. White foamy stuff comes off the surface of the aluminum.

The third photo shows what it looks like when it comes out of the goop. It has an odd sheen and slightly pink tint.

The fifth photo shows it after it was rinsed and toweled off, air drying.

The last photo shows the crud that settles to the bottom of the rinse water. It's probably the same crud in the etch tank. I am curious what it is.

I'm no chemist and I'm not interested enough to do much research, so take this explanation with an unhealthy dose of salt. I "think" the phosphoric acid converts the aluminum or aluminum oxide or both to some chemical that the paint will adhere to better. Don't quote me on that. But what I do know is it works. My experiments have shown that doing this entire procedure but without this etching step makes the paint stick better, but nowhere near as well as with this etching step. That makes me believe this is the most critical part. I think this falls in the category of "conversion coating."

Once the part is finished soaking, take it out and drop it into another container of clean water. Again let it soak for about 30 minutes or so. You will see a lot of stuff dissolve into the water. Then remove the part and rinse it thoroughly under running water. Dry it off gently with a clean paper towel or two. Note the look of the surface. It will usually be a dull whitish-gray with perhaps a very light pink tint. And it should be uniform across the surface. Let it air dry really well. I find placing it in front of a fan helps a lot, propped up where air can get all around it. Once it is dry it is ready to paint. It is probably a good idea to not wait too long before painting it; we just put a lot of time and effort into cleaning and getting rid of oxide, so why let it get dirty and oxidize again? I usually try not to wait more than a few hours, or a day at most.

Using self-etching primer, spray the item with light coats. Make sure to cover the entire surface, but do use light coats. You can spray additional coats after about fifteen minutes. It usually dries to the touch pretty quick. I generally use three or possibly four light coats of self-etching primer. For a nice, smooth surface, I will let the top coat dry for a day or so and then lightly sand it with 400 to 600 grit paper, or maybe even 1000 grit if I want it really smooth.

In the photos you can see the two different primers I use. The one from Harbor Freight is grey and the Rustoleum one is green. I have more experience with Rustoleum, but so far have not noticed any difference between them as far as how well they work.

The paint booth is my own design. I'm still waiting to submit my patent application.

If you read any old methods (including the one used as a basis for this instructable) on painting aluminum, they always say to use zinc chromate paint. I haven't yet found any zinc chromate paint that is easily available, and reading up on the stuff I'm glad I haven't. It's really nasty stuff. I suspect that modern regulations have limited its use. All the self-etching primers I have found readily available use zinc phosphate. I have found it to work very well. Maybe the zinc chromate works better. I have no idea. If you can get it and don't mind the hazards, give it a try.

After a couple of hours to a couple of days of drying time, you can spray it with whatever top coat of paint you want. I like to use epoxy paints for anything that might get rough handling.

After a few days, try what I call the "fingernail test." Try to scratch the paint off with a fingernail. It takes a few days for the primer to cure all the way, but after that I find it stands up very well. With any other procedure I've tried, I was still able to fairly easily scratch the paint off even after weeks of curing time. With this procedure I can't. I can usually scratch into the paint, but not scratch it off to bare aluminum.

You're done! I hope you find this process valuable. It works well for me and has saved me a lot of time and aggravation.