In the aircraft industry we first prep the bare aluminum with Alumaprep solution from any Dupont distributor or any equivalent weak phosphoric acid etch solution to clean and etch the surface. Then we apply alodine which is a conversion coating that "pickles the surface and makes it resistant to corrosion. This alco can be purchased from Dupont distributors. In the old days we would prime the dry, previously etched & alodined surface with zinc chromate primer but since it is a known carcinogen a good epoxy phosephate or chromate primer would be an excellent substitute. We use Super Koropon but even that is getting hard to get. Any good quality epoxy primer will do in a pinch. You can apply your base coat over that.
I would bet on HOK Epoxy Primer. They have a version with or without chromate. Prep like you would prep bare metal. 40 grit on a DA, blow gun, tack cloth, shoot.
The alodine solution is an important step. Aluminum needs special consideration for the following reason. Aluminum is one of the most reactive metals. When you sand or grind it the very top atoms on the surface immediately combine with oxygen to form aluminum oxide. Aluminum oxide is a very hard slippery material that does not adhere well to paint. The alodine solution "converts" the top surface molecules chemically to a material that bonds to paint more readily. To just grind and sand prior to priming is just asking for trouble.
I manufacture aluminum windows and I use Sherwin Williams Wash Primer P60-G2. I clean them first with an etching industrial soap to remove saw cutting oils and fingerprints. I have never had the paint peel from one of my frames and have been in this business for 10 years. Some of my topcoats are plain, quick dry enamel and some are Polane T.
The OP asked for the "correct" method. My first experiences painting Aluminum were back in 1981. Perhaps information from the following webpage will be interesting: http://www.ronjoseph.com/Q&A/F1001_primer_vs_coating.htm
I didn't know there was a "correct" way as long as the paint sticks and doesn't peel off. What should it matter how you prep before painting. As far as I know hot rods and windows don't have to meet Mil Specs before painting.
I've just painted an aluminum body (handmade MG TC). The paint shop told me to scuff/sand the aluminum and shoot it with high build primer - nothing special. Finished with acrylic enamel and clear coat. I screwed up once and had to re-do a part of the body...paint stuck like a bitch...hard to get off.
What is your favorite brand of paint?Now contact that manufacturer and ask them this question.Tell them exactly what it is you are painting and how it will be used.then follow their advise exactly and use only their products through out.I'm not trying to be an ass here or flame you or your post.Each manufacturer you talk to will tell you that their process "Is the only way"to properly paint this or that material.And they are most likely right USING THEIR materials.I ran industrial paint departments for 2 different companies for a total of 7 years and in one shop we painted aluminum structures used in swimming pools for physical therapy treatments.We used Sherwin Williams Industrial Coating at both companies.The paints used in the underwater applications on aluminum was rated for 1000 hours of continues submersion with out failure.Sherwin Williams mixes paint for over 30 different paint brands so chances are good if your using some other brand Sherwin Williams made it.just for the record BEFORE I get flamed I think BOTH these guys are right for THEIR trades.With that said your street/hot rod will most likely never do 300 mph in bad weather!And aluminum window never get parked inside for the night!CAPS aree for emphasis only not intended in any other way
I live in South Florida. I have seen a large amount of commercially manufactured aluminum products with their "factory coatings" peeled off. Of course our climate conditions are a little more brutal than most. If someone is painting parts for their personal rod I would think they would want the best method. The alodine process is not really that more expensive from a materials aspect, but does take a little more labor. If nothing else the extra adhesion would provide better performance from impacts of small objects (stones, large bugs, small birds etc.) and just in case you did manage to get your rod above 300 mph. Most of my experience with painting aluminum came from building electric signs. One aspect is that some of them have service panels that require fasteners that were cycled many times in the lifetime of a sign. We were less likely to lose paint from these parts using the alodine conversion process.
Wow, not to knock your experience, but that is way in depth. One piece of advice I will give you on here is that many people (flathead okie not included) are looking for the way which will get them average results with the lowest cost and average labor time. Granted your method is probably A OK and will give the best adhesion for 50 years, the fact of the matter is that might not be what he/she was looking for. The primers I use have their tech sheets calling for 40 grit on the substrate (aluminum included), clean and spray. This works because I have done it. The adhesion I achieve may not be as perfect as yours, but I have had no complaints. Not to say that I am perfect either, but I have given poeple advice on (to the best of my knowledge) the right way to prep/paint a car and watch as they take the easy way out. Then they ask why. Its just the cycle around here. Give your advice on your method and let nature take its course. Keep in mind that there can be more than one way to do things.
How 'bout calling a body shop that does alot of semi-trucks (K-W, Pete's, IH)? Most of the panels/doors/sleepers are aluminum.......
At work (MEDICAL EQUIPTMENT OEM) the paint spec for aluminum USED to be to first have it sent to a plater and finished with "chromate conversion" prior to painting it with Polane. I had some scraps sitting outside in the weather for years - the shit was amazing. Matter of fact Polane in general was amazing. I can remember being on the assembly floor pounding in dowel pins and ocassionally missing them with the hammer. WHen the hammer didn't squash my thumb and hit the "good" part that freakin polane NEVER EVER even chipped - You could dent the hell out of the metal - that Polane was there. Made me think they knew what teh hell they were doing!!! Well then along comes the bean counters, tree huggers and the rest of the parade. Now you can blow off the parts with the air hoes and see the paint fly off....what were we talking about again??? Oh yeah painting aluminum the RIGHT way. Well as other posters have implied - what is your definition of RIGHT??
The reason I use the Sherwin Williams P60 G2 is because the reducer used for this primer contains a phosphoric acid (as well as other chemicals) that etches into the aluminum creating a very tight bond making it virtually impossible to remove. I have even used it on mild steel with great results. I would not use it for automotive use but for anything else, it should be OK.
The Jalopy Journal
