preping raw polurethane plastic what did i do wrong ?

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PostPosted: Sat Apr 13, 2019 12:32 pm
Bought a Raw Polyurethane Plastic Bumper I preped it and painted it after two weeks paint started cracking and I don't know what to do or where I went wrong here are the steps I did

1) Clean and use wax and grease remover.

2) Sanded the whole bumper with 320grit with hand

3) Applied Transtar Aerosol Can Of Flexible primer

4) Sanded primer with 400

5) getting ready for paint we applied BullDog Adhesion Promoter then spayed basecoat then clear coat.

after 2 to 3 weeks bumper started showing stretch marks, please any help well do or where I went wrong

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PostPosted: Sat Apr 13, 2019 3:51 pm
Well, I think you have a chemistry problem here as well as having applied 2 flex agents. If you look up the spec.s on Bulldog you'll read that it doubles as not only an adhesion promoter but FLEX agent, too. The other thing about Bulldog is it has 14 very "hot" solvents in it which means it is best used on raw plastics, not up in your coating process. It should have been applied on the raw plastic, then primers/sealers, and on to base coat/clear coat. The Transtar flex was not needed....... Most base/clears are so flexible now that you rarely see a flex. agent being needed in most paint systems.
Last edited by DarrelK on Sat Apr 13, 2019 3:52 pm, edited 1 time in total.
Reason: spelling
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PostPosted: Sat Apr 13, 2019 6:33 pm
so you think is because I didn't apply the adhesion promoter to the raw plastic?

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PostPosted: Sat Apr 13, 2019 10:52 pm
Yes, it should have been on the raw plastic and there was no need for that flexy style primer. A primer with the flex in it was not needed.
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PostPosted: Sun Apr 14, 2019 11:31 am
So no primer was needed , just adhesion promotor then base coat clear coat , or should i apply something else ! Sealer ?

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PostPosted: Sun Apr 14, 2019 10:16 pm
The order would have been adhesion promoter, primer (no FLEX primer) or sealer, base color, then clear. Here is a small job I did on a bumper. In the spots where I did go down to raw plastic I did hit it with promoter. The filler was just barely needed to smooth out the surface. viewtopic.php?f=13&t=27152
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PostPosted: Mon Apr 15, 2019 4:14 pm
what should i do sand the bumper back to raw plastic if so what grit should i use ?

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PostPosted: Mon Apr 15, 2019 9:32 pm
Not coarser than P180 and then finish to P400.

But.....

I think that the issues may run deeper than just the wrong use of primers, flexible or not.

We have no photos to assess but paint doesn't just crack by itself. To crack a urethane clear you need quite a lot of movement in the substrate, particularly when the clear is still relatively fresh and not completely hardened.

Mismatched chemistries are more likely to manifest in surface imperfections like solvent pop or wrinkling. The "stretch marks" referred to by OP may be more indicative of an adhesion problem or a case where the base layer hasn't hardened while the top coat has. For example, an insufficiently hardened primer may stay soft underneath while apparently hard on the top. The use of adhesion promoter over a fresh new layer of paint may result in similar softening.

IMHO this needs to be analysed a bit more and the real reason established before the work, and the problem, is repeated. That said, sanding off what is there now should give some good clues if there is a layer there that is causing the problem.
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PostPosted: Mon Apr 15, 2019 10:15 pm
I was thinking probably doing is sanding it down back to raw plastic and adding ppg adhesion promoter then putting poly primer then base and clearcoat what you guys think?

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PostPosted: Mon Apr 15, 2019 10:21 pm
It indeed could be we are not interpreting what "stretch marks" are here. Any chance at all you could put up a pic. or two? Definitely agree with you on sanding back....going back to the plastic would be a clean slate approach to trying to work it out.....
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