Old Ham Wisdom Leads To Better Aluminum Painting

When [bdk6] tried painting aluminum for electronic projects, he found it didn’t tend to stay painted. It would easily scratch off or, eventually, even flake off. The problem is the paint doesn’t want to adhere to the aluminum oxide coating around the metal. Research ensued, and he found an article in an old ham radio magazine about a technique that he could adapt to get good results painting aluminum.

Actually, paint apparently adheres poorly, even to non-oxidized aluminum. So the plan is to clean and remove as much aluminum oxide as possible. Then the process will convert the aluminum surface to something the paint sticks to better. Of course, you also need the right kind of paint.

The key ingredients are phosphoric acid and zinc phosphate. Phosphoric acid is found in soft drinks, but is also sold as a concrete and metal prep for painting. The zinc phosphate is part of a special paint known as a self-etching primer.

Cleaning takes soap, elbow grease, and sandpaper. The next step is a long soak in the phosphoric acid. Then you apply a few coats of self-etching primer and sand. Once it is all set, you can paint with your normal paint. That’s usually epoxy-based paint for [bdk6].

Of course, you can also dye aluminum while anodizing it. Soldering aluminum also has its challenges.

50 thoughts on “Old Ham Wisdom Leads To Better Aluminum Painting

  1. “…even to non-oxidized aluminum.”

    That’s because there is no “non-oxidized aluminum.” Oxides form much too quickly to ever do anything with “bare” aluminum. The surface will be aluminum oxide.

    1. Well, if you take the oxide off and keep it under something like oil you can keep it from oxidizing. Now I’m not sure how you paint it and that condition. Or perhaps you painted a vacuum chamber or under nitrogen lol

        1. Post oxide removal nitrogen or vacuum aluminum painting! That’s probably still better than epoxy “painting”.

          Or just use an actually adhesive spray paint and move on if you want to be quick and avoid epoxy.

          Anodizing seems awkward given it is a cast aluminum.

      1. I was once at an aerospace company that was using some kind of exotic alloy that would instantly oxidize in air. They had the setup I have ever seen.

        Imagine a machine shop built in an empty swimming pool covered with giant sheet of kitchen film. The ‘pool’ was flooded with nitrogen and there were two machinists working down there with breathing equipment plugged into house air.

        Completing the pool metaphor was the guy in a lifeguard chair whose only job was to watch the workers for signs of distress and hit a ‘scram’ button that would flood the area with fresh air, rupturing the film and blowing out the nitrogen.

        1. Machining metallic actinides and their alloys requires similar care; some rapidly oxide – explosively – when the fresh surface area is increased in a short time. Bonus hazard: airborne radioactive particulates. So they need to be worked in an anoxic environment, also away from chlorine since that element also supports a process similar to spontaneous oxidation(combustion).

          This as a serious problem and led to several serious accidents at Oak Ridge and Hanford.

  2. You can’t solder to aluminum without cleaning it carefull and keeping air off it. It oxidizes otherwise. The old ham magazines had articles about doing this.

    I wonder if it’s related?

    1. See the link. You can solder directly to old aluminum: when the linked article went live I tried it and it worked. It’s not a great solder joint, but it’s solid enough.

  3. Aluminium is interesting. Refractory oxide that is tightly adherent and as hard as sapphire- IS sapphire, actually- but is also very susceptible to insult from impact or abrasion exposing the metal underneath. The metal itself is SO reactive that you need to work to expose sufficient area to bond.

    Back in the day, the technique we used to epoxy aluminium in the optics lab was wet sanding, using the resin as the wetting agent. This got the resin into intimate contact with the bare metal before it could reoxidize. Then wipe off and bond immediately after with a proper mix. Demonically strong adhesion.

    For painting, we used a commercial phosphate pretreat, again wet-sanding. Worked a treat, and I haven’t thought about it for a number of years.

    1. Interesting. I wonder if you could use almost any paint, like may e an acrylic, “wet sand” with it. Since you’re cleaning off the oxide, and its already under “something” to keep it from re-oxidizing instantly, you can just wipe it off (leaving just enough), and then apply a final coat or two.

      I’m guessing “no”, because it just sounds too simple.

        1. I think the quote marks around “wet sand” were there to indicate that no water is involved: the “wetting” (lubricating) agent is the non-water-based paint or glue you’ll be using a couple more coats of. The thin layer left over when you wipe off the excess sand will prevent oxidation while you apply those coats. As I understand it.

    1. Have you ever seen a paint can lately and read the instructions?
      I haven’t, or at least not for a long time and a long time ago there were no clear instructions regarding every possible material, just the basic instructions of cleaning, priming, drying, more drying in various languages and in a horribly small print (which is probably even horribly smaller now I’m a bit older and constantly reminded of the fact that I need glasses).

        1. I miss being able to push my -5.50 right contact lens aside and focus on tiny print on microchips at a distance of about an inch. My left eye takes a more modest -3.00 correction.

          I can read books without my contact lenses, just can’t see anything clearly as far as my outstretched arms. With contacts in, I can see *way over there* just fine but anything within arms’ reach is blurry.

          Would be nice if those Chinese eye drops that restore the lens’ flexibility would get USA FDA approval.

          1. I’m -4.75 / -4.50 and wear glasses, so yeah being able to just lift them out of the way to examine tiny things up close was awesome.
            Then I hit 45; now I wear bifocals and even 3 years later can never manage to focus *anything* clearly (feels like). #FML

    1. Came here to write that. Powder coated aluminum is incredibly durable. It is fairly easy to do at home if you have an oven that you can use (ie not your kitchen oven). You can get powder coating guns for less than $100. I’ve had excellent results powder coating front panels and then laser etching labels into them.

      1. Funny you should say that. Literally just a few weeks ago I was cleaning a powdercoated aluminium part that had spent the last few decades in a garage shop on a shelf (case in point, a certain kind of temperature and humidity swing over the seasons, but overall not a very hostile place).
        Lacking a media blaster, and since no chemical I could get my hands on did a thing to it, my third thought was heat, which worked but was extremely messy. After a bit of thinking I thought “well I might just try peeling it”. Literally peeling. With a pocket knife. Like an apple.
        It worked incredibly well.
        Granted, casting, powder coat and workmanship were probably all products of ex-Soviet Bloc industry, but – based on a whopping number of this single one specimen – I’m not sure I’m calling this ‘incredibly durable’.

      1. The trademark name is spelled ‘Alodine’. Otherwise chem-film or chromate conversion. Yes, the hex chrome is not used any more.
        The new version, Type II, uses trivalent chrome, and satisfies RoHS and OSHA regulations.
        Around these parts, the product is TCP-HF, from Chemeon.

        1. @Paul said: “Around these parts, the product is TCP-HF, from Chemeon.”

          Aerospace manufacturers and contractors in the U.S. use Chemeon TCP-HF by the barrel full. It’s not cheap, it is fairly safe to handle, properly stored it has a two-year shelf life, and it is hard to buy unless you need a lot of it. Companies that specialize in coating and plating contracts will sometimes offer small batch Chemeon TCP-HF coating services, especially if they serve the aerospace industry. Shop around.

          Chemeon TCP-HF® is a proprietary chemistry developed by Pratt and Whitney Aircraft®, one of the world’s leaders in the manufacturing of civil and military aircraft engines and as such, this technologically advanced product was tested against the most rigorous standards used in high performance chromate conversion for aerospace applications.

          * CHEMEON TCP-HF – A chromium (III) conversion coating used for immersion and spray applications which is Mil Spec, ROHS, WEEE, and ELV compliant.


          * CHEMEON TCP-HF


          * Chrome Plating – Trivalent Chromium


          * CHEMEON TCP-HF – Where to Buy [No Affiliation]


  4. Self-etching aluminum primer is inexpensive and has been commercially available off the shelf for decades. I used it over 20 years ago to treat an large aluminum box I had built before painting it. It sits outside in the weather, I’ve never need to repaint it since, and I live in a brutal environment for paint.

  5. How about vinyl wrap instead? Seems the easiest quickest and cheapest and least risky way.
    Or are you now going to tell me the glue won’t stick on oxidized aluminium?

    BTW, isn’t that how they do it with aluminium cans? Some sort of film and then heat-bond it? I’m just guessing here so please correct me if I’m wrong.

  6. Note to the HaD staff, I noticed that many food packaging is actually PE/PP and yet has illustration and lettering on it that is very robustly attached. It looks like they melt in the text/graphic somehow, and interestingly you can paint those plastic with such applications on them, without it peeing off immediately like with regular PE/PP.
    So if you could find how to treat PE/PP that way at home it would make an interesting article since even without text and illustration applied you can make PE/PP paintable after such treatment.

  7. I once cleaned an old trolling motor aluminum head, after cleaning it with white vinegar. Just spray painted it with black paint straight from the can. Two days later I scratched tested it and was sorely disappointed…. Left it alone for two years (don’t worry, I still went fishing, using paddles…) and when I finally got back to it, that paint was not catchable. Hard as nails. And the two years old test scratch was just starring at me, oh, so noticeably, lol!

    Just let it dry com-ple-te-ly might be the trick?

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