Ask the Expert Question-and-Answer Archive

by Ron Joseph

July, 2003

Painting Aluminum

Q. I make cold air induction systems for cars and I make them from 6061 aluminum. I get the aluminum in a raw state. I cut, sand, rinse in water, dry, acid etch prime and paint with engine enamel. The paint chips at the drop of a hat. What can I do to fix this? I need to maintain a wide range of color availability and heat resistance. I do not have the capital to invest in anodizing or powder coating at this time. I would rather have a durable paint job. Any suggestions? These are under the hood of a car and the temps can reach 200-300 degrees Fahrenheit. Any info you can offer would be greatly appreciated.

A. From your email I can't figure out if the paint is too brittle, or if the paint is not properly adhering to the substrate. Engine enamels should be able to take the elevated temperatures because they are formulated for this specific purpose. My first approach would be to look at the surface preparation. Acid etch primers must be applied to the correct film thickness. You must have a dry film thickness of between 0.3 - 0.5 mils. If the primer is applied too thick, it will spilt down the middle of the film and the paint above it will come off.

Ideally, I wouldn't use a wash primer at all. There are conversion coatings that can be applied by spray that might be better. Most conversion coatings are applied in multistage processes, and it is not unusual for an automotive assembly plant to have a 7-9 stage process, including several rinse stages. By contrast, you are applying a simple coating in a one-step process which might not be adequate for the application. Another thought is that while the engine enamel should have been formulated for the elevated temperatures, the wash primer might not be able to withstand such high temperatures, and this might be where your problem lies.

If you decide to continue to use the wash primer, insure that you are using a formulation that is specifically intended for aluminum. Usually, a more dilute acid concentration is used when the wash primer is applied on a metal that is less reactive than steel. Aluminum is one such metal.

For proper dedicated conversion coatings intended for aluminum you might like to visit the site for Henkel Surface Technologies, http://www.buyhstna.com/productuses.htm and evaluate their inorganic chemical products. Also, you might give their technical people a call and ask which products you should consider using. You can even ask the folks at Henkel how well the 6061 alloy will perform with the conversion coatings. Please don't underestimate the effect of the alloy material itself on the degree of adhesion you can achieve from the various chemical products. All alloys behave differently.

Follow-up Q. You are correct, it’s the adhesion I am having problems with. These chipping problems happen before the item has even been put into service. I will check that site you gave me. I am currently using Eastwoods Acid Etching primer intended for use on steel or bare metal. I am a small operation and use mostly spray cans so I am looking for a cost effective way to paint these. I am going to look into the conversion coatings you referenced to. I do not know how to tell how thick the film is on the surface is there something I should be measuring this with? I was contemplating coating these pipes with a clear epoxy in an effort to give the surface more elasticity. Do you think that would work for surface protection or is epoxy not very heat resistant? Also, what can I use to remove the oxidation from the aluminum in preparation for painting? I was told rubbing alcohol or acetone. Is this true?

A. You can purchase a dry film thickness gauge for nonferrous metals from companies, such as www.gardco.com, www.defelsko.com , www.bykgardner.com and more. The instruments are not inexpensive (approx $1,200), but you can calibrate them quite easily. Epoxies can become very brittle. I would not use an epoxy unless i knew that it would adhere well to the aluminum and would not crack after it has been exposed to the hight temperatures for lengthy periods.

You can only remove the oxidation with physical abrasion or with very strong acids that are contained in the conversion coatings. No solvent will remove the oxide, nor will the acid etch primer.