I posted this in the sticky thread "Let's take the guesswork out of plastic repair" but haven't received any type of response so I thought I would just post a new thread.

My poor chin spoiler has seen better days, so I bought a used one a while back and started prepping it for paint. I'm not a paint/body guy, but I've preped my old chin spoiler a half a dozen times at least to get it repainted. With the "new" bumper I found a surprise underneath the paint this time. started sanding it down though the typical spiderweb cracks they all get after they've bumped a few curbs. The cracks on this bumper go all the way through the paint into the actual bumper. I've never seen this before, but maybe some of you others have.

My question is, is there something that can be put over the spoiler to seal these litttle cracks up so it won't crack new paint? or do i need to try to sand them all out of the polyurethane? They aren't just on the surface, but they aren't incredibly deep either. However, I'm a little bit worried about keeping it even if I have to go the sand 'em all out route.

Any input appreciated.

Here are a couple of pics:





And a close up:

 

from my experience they will come through the paint and i would sand them out first even cracks in the paint it's self will come through.

 

I know you should use a flexible primer to keep the cracks from happening. I'd sand em out as fine as I could then use the flexible primer. Then of course sand...primer...sand....primer process. If you can get em to where there not so deep I'd say they wouldn't come through, but by all means I'm no professional body man.

I just fixed a tear in my spoiler and repainted. I didn't have alot of cracks but I had a couple deep pits, like a scattergun shot.

 

I have fixed many bumpers like this. I just kept priming and priming...and they turned out fine. Using K36/K201 by PPG, great high building primer.