i'm posting this here because the cosmetics forum is useless. if it isn't appropriate, feel free to move it.

i added some brake ducts to my crx a while back. right now i have cutout holes (jdm bumper, where the fogs would go) filled with naca ducts that have been modified (heated, bent, and cut to fit) and riveted in. they look ok, but the car is getting nicer (and painted) so i want to smooth everything out and make it look as professional as i can.

the bumper is also scratched pretty deeply in lots of places, and the trim has been sanded.

my question is-

what is the best thing to use to bridge the voids (1/8 in max) and flush out the ducts? i was thinking fiberglass mat, then bondo.

and what is the best way to smooth out the bumper? can you skim coat it with bondo like a metal part, or is the plastic going to cause problems due to adhearance or flexing in the future?

thanks in advance.