Aluminum corroding under clear coat

[pdeinc]

I have sucessfully sprayed clear over a variety of polished and tumbled aluminum parts, including a set of valve covers. However, within a couple of weeks I could see corrosion occuring on the valve covers. None of the other parts are doing this, and the corrosion actually started before the engine was ever fired up and appears to be speading steadily.

Has anyone else experienced this? And what would you suggest to do before recoating to prevent it from happening again?

[Blademan]

see if you can find zep-a-lume its a aluminum cleaner, it needs to be diluted as instructed and do not leave the part in for more than 3 to 5 min.
then rense really well. it is an acid so be careful, I wash all the parts that I do for Freeman Marine, hatches, handles,port lights and never had any corrosion..a good lumber yard should have it.

[pdeinc]

Thanks for the response, but won't the acid wash dull the polished finish?

[Blademan]

yes it will, I was suggesting you wash first then polish.

[pdeinc]

That's pretty much as I have done it. the cleaner/degreaser I use before spraying also has a mild acid base, but not enough to dull the finish. I guess I'll just have to strip them and do it over.

I preheated them last time and think I won't this time to see if it makes any difference. None of the other polished aluminum parts got preheated and they don't show any signs of corrosion.

[Blademan]

you may be onto something there,i never pre-heat my alum unless its an old engine part.

[pdeinc]

Well, that was actually the second powder coat I had put on it, as the first one got contaminated from outgassing of the part during the first cure. And they are 'old' engine parts, valve covers from a '65 vette.

So I stripped that coat off and preheated the part before spraying the next coat. It appeared to work well that time. It was a couple of weeks later that the corrosion started to show up.

I also did an alternator housing the same way - had it polished, cleaned it well, preheated and sprayed it, no corrosion. Something about those valve covers tho...

[Blademan]

I have an intense dislike for old valve covers................

[pdeinc]

I could start to see why too... But considering the value of these, I'll be sticking with them.