Spraying heavy primer

[gtome]

So I was spraying the inside of the doors of this car yeasterday. The car was not stripped, and it old, so There are chips and the like.
Anyway, I noticed that I seem to spray 2k primer in these areas pretty heavy, as a sort of spray filler. Never really paid attention before, but now after thinking about it, I believe I do that from time to time. Is this bad? I give it good flash times, and I sand a lot back off. Just wondering if there is a downside to doing that?
I really dont see an easy alternative on the cars that arent stripped first.

[88GT]

Ive heard that refered to as "primer abuse". Its not prefered, but I think we've all done it a time ot 2. It may or may not be ok.

[Steve g]

So I was spraying the inside of the doors of this car yeasterday. The car was not stripped, and it old, so There are chips and the like.
Anyway, I noticed that I seem to spray 2k primer in these areas pretty heavy, as a sort of spray filler. Never really paid attention before, but now after thinking about it, I believe I do that from time to time. Is this bad? I give it good flash times, and I sand a lot back off. Just wondering if there is a downside to doing that?
I really dont see an easy alternative on the cars that arent stripped first.

One downside to it is that it's an incredible waste of product. If you are not removing the old finish you're not accomplishing anything by putting high build primer on it. It's purpose is to fill. What are you filling on top of the old finish? If you have chips and abrasions you want to fill to level the surface again you want to feather those areas out to the surrounding area, build those areas only up, then sand them level with the surrounding old finish. In areas like jambs it's even questionable how much you need to level the repairs. In most cases just feathering them out and hitting the exposed metal with epoxy will suffice.

Higher film builds will also chip easier and make a deeper bruise when they do chip.

It seems to be a popular misconception that new paint needs primer under it. The need for primer is not determined by what's going to be put on, but rather what it's being put on.

Steve g

[gtome]

Most of it gets sanded back off. But really I do that where it would take me hours to sand out difficult to reach areas. Obviously not the ideal way to do it, but it does get it done quickly. Mostly it is just filling the chips and other uneven areas. Seems like i always end up doing that on the inside of the doors around the bottom lip, or other hard to reach areas.

[Steve g]

Most of it gets sanded back off. But really I do that where it would take me hours to sand out difficult to reach areas. Obviously not the ideal way to do it, but it does get it done quickly. Mostly it is just filling the chips and other uneven areas. Seems like i always end up doing that on the inside of the doors around the bottom lip, or other hard to reach areas.

I'm not sure how you're saving any time. 2k urethane primer requires a good scratch to adhere to so a thorough sanding is required before you apply it. Wouldn't it be easier to take 180-220 paper to your flaws only and feather them out, shoot your 2k over just those areas only then sand the whole thing with 400. it would seem to me as it stands now your're sanding everything twice instead of the repair aeas once then the whole thing once. If you're applying your 2k on these areas without a good sanding you're at high risk of it falling off.

Steve g

[gtome]

Yeah, I am a little worried about good adhesion. But I have to sand these by hand, and usually use 80 grit with my fingers. Maybe I should post a pic of where im talking about.

[Steve g]

Yeah, I am a little worried about good adhesion. But I have to sand these by hand, and usually use 80 grit with my fingers. Maybe I should post a pic of where im talking about.

Primer, especially a 2k urethane, is not a substitute for sanding. If I have an area that I can't sand well I would scuff it as best I could and use an epoxy primer followed by the 2k if it needs building. If it just needs paint then a scuffing and an adhesion promoter before your topcoat would be all you need. But 2k urethane over an unsanded area is almost certain to give you problems.

Yes, sometimes these places are not easy and take time, but if you want it right that,s what you do. I actually do most of my sanding by hand. I do large scale removal by machine, but all the blocking and prep in the areas you describe is done by hand.

Steve g

[Phil V]

I use acid etch primer in areas that are difficult or impossible areas to sand. The acid in the acid etch actually eats into whatever you spray it on including areas previously painted. Its not great for filling imperfections but its great for adhesion. If you needed filling of the 2K high build primer then spray the acid etch first then the 2K urethane primer over that.

Its definately not a big deal if you spray 2K primer a little thick on the bottom of doors etc. My experience is that it won't cause any long term problems.

[Steve g]

I use acid etch primer in areas that are difficult or impossible areas to sand. The acid in the acid etch actually eats into whatever you spray it on including areas previously painted. Its not great for filling imperfections but its great for adhesion. If you needed filling of the 2K high build primer then spray the acid etch first then the 2K urethane primer over that.

Its definately not a big deal if you spray 2K primer a little thick on the bottom of doors etc. My experience is that it won't cause any long term problems.

I'd never heard of using etch as an adhesion promotor. The etches I've used, Dupont's Variprime and Omni's etch show .acceptable substrates as bare steel, aluminum or galvanized. Omni specifically warns against getting it on filler, but doesn't mention anything about topcoats. I would have thought the plastic nature of the finish would be unaffected by acid, similar to the plastic containers they ship acids in.

Acids bite into metal, solvents into resins and plastics.

Steve g

[another2centsworth]

2 etching primers: self etch, and Wash primer
think etch (Matrix) is ok over filler, bare metal, or scuffed paint.
what I have heard is wash primer (PPG) wont work on filler. mostly for bare metal.
Used wash on bare steel 15 yrs ago.

[gtome]

Damn! I never even thought of useing adehsion promoter there. That would be good piece of mind.