I experimented this weekend, after trying to use Bondo on some ABS wheel pants which failed miserably. After roughing the surface thoroughly, I still had a hard time getting Bondo to stick to the surface of the ABS parts. It sticks great to metal, as it was made for, but not to plastic.

I needed something better. Spackle is no good, as these plastic parts flex a little, and spackle easily cracks. So, I tried a product made by DAP. Used it for making cabinets before.

It is called "Plastic Wood", made by DAP. And it works GREAT!!! Especially for plastic parts. I used it sparingly, as it is heavy like Bondo. But it really sticks well, and is more flexible than spackle. After letting it dry for several hours, I sanded this stuff so smooth that it blended right in, and was much nicer than the Bondo. It takes primer and paint with no problem. I am going to use this from now on. It is so much easier to work with compared to Bondo, and doesn't STINK either.
I bet this stuff would work beautifully on fiberglass parts as well, it just seems to stick to everything.

Patriot