Re-coating over out-gas bubbles?

[pdeinc]

Clear coated over a pair of polished aluminum valve covers last night with mediocre results. I felt I had adequately cleaned and de-greased them and then pre-heated before spraying. The first one had minor defects here and there except for the top surface which looked like sand had been sprinkled over it!

The second one had several areas with what I had have to guess was more out-gassing. That was interesting because after the poor appearance of the first one, I passed the second through another cleaning operation and then pre-heated it longer and probably sprayed it a little sooner after pulling it from the oven.

I'm afraid that stripping the coating entirely is going to result in damage to the finish requiring re-polishing again. And I'm not so sure that the polishing compounds that might be imbedded in the metal aren't part of the defect problem in the first place.

Now my question is do I have a chance of re-coating these with a second coat after sanding or steel-wooling the defects? I'm fairly confident I got a adequate coating thickness on it so I feel I can go over these defects without going thru to the metal. Obviously they won't be removed, but if they're smoothed out sufficiently, a second coat might diminish their appearance for an acceptable overall finish.

[Noob2PC]

What I end up doing is sanding down the outgassed area and reshot the whole thing. I have been outgassing or pre baking all my Aluminum parts for 40 min at 450. I will be doing the same for my intake once I get it back from the machine shop

[pdeinc]

Your response is interesting in that I have begun to suspect that the recommendation in the "tips & tricks" of pre-heating at 250 degrees is inadequate. If the part is not pre-heated above the temperature at which it is to be cured, further outgassing can and will occur.

I performed an experiment last night with my intake manifold. I should mention that it had been degreased 3 times and washed thoroughly before this test. I pre-heated it first at 250 degrees for 20 minutes and took it out and examined it. I found brown stains in several spots indicating outgassing of contaminants. I solvent cleaned these areas and then preheated it again at 450 degrees. Examination showed additional outgassing had taken place. Cleaned it again and preheated back to 450, then removed and coated it before curing at 400. No stains, no bubbles, looks great!

One problem I did encounter is that when spraying it at these elevated temperatures, the powder melts immediately and it's difficult to gauge how much powder you've applied. How far do you let it cool after preheating before spraying?

As for re-spraying my bad parts, I'm having reasonable success steel wooling many of the defects, but there's one large area that doesn't lend itself to that process. It's an unpolished part of the valve cover so I've tried bead blasting that area to simply remove the bad PC but as you probably know, bead blast media is not abrasive enough to do the job. I may have to do a media change-out in my cabinet but don't relish that job either. I've tried a fairly strong paint remover with no success and have heard mention of an 'aircraft' paint remover for PC removal. Do you or someone have more information of brands or sources for this?

[pdeinc]

Two pieces of new information to add to this thread. First, I found a stripper that removes PC fairly readily. It's called Zip and says it's for paints, clear finishes and other marine applications.

Second bit of news you can probably guess. Used the stripper to remove the PC from the valve covers, re-polished (didn't take much) and then pre-heated to 450 degrees. Pulled them out and solvent cleaned whatever contamination that appeared. Let them cool down a bit and then resprayed. Then baked at 350 degress for 20 minutes, rather than 400 for 15. Came out looking great!

[jtagger19]

aftre I pre-bake at 450 , I let it cool to 150 and then powder it
if doing second coats let the first one cool to 150 again and recoat
it has never failed me

[Noob2PC]

Glad to see you worked out the kinks. I spent all day stripping, wire wheeling, and blasting a ton of parts that will be PC'd over the week if I get time.

Where did you find Zip at?

[pdeinc]

Found Zip at my local Ace Hardware store. Did some more aluminum with the same process again with good results. Someone needs to update the tips and tricks to mention preheating to higher than cure temperature. I see that is also mentioned in a PC manual referred to somewhere else.

[Blademan]

it aint cheap...but i use napa's carb cleaner for stripping...also been known to use muratic acid (dilluted and make sure you add the water to the acid ) and watched very carefully...i hate blastin off pc and i dislike chemicals like aircraft stuff even more.

[pdeinc]

Funny you hate chemicals but use muriatic acid! I have gallons of it around for my pool and have used it to clean metal, but I didn't realize the muriatic would react with the PC.

In this particular case, it wouldn't have been a good choice as it would have destroyed the polish on the aluminum, whereas the stripper pretty much left it untouched.

[mkewl]

What is outgassing?