Bondo-Hair will stick to the plastic surface much better since it is basically fiberglass resin, strands of fiberglass, and a little bit of bondo filler mixed together. The first time I filled my bumper cracks I used Bondo, drove my car for a little over a week, and had cracks forming. I sanded it off and replaced with Bondo-Hair and no cracks ever since.
As far as the finish do like what was mentioned before and use regular bondo filler to smooth out the finishing surface. That's how professionals usually use it. I remember that's also how custom car shops build custom door panels. (Stretch Cloth over panel skeleton - Paint w/ Resin, form with Bondo-Hair (short hair), Sand, Smooth out surface with regular Bondo, Sand till smooth, then paint)
Also know that Bondo-Hair sets up harder to sand then regular Bondo so try to start knocking the high spots off it right before it's completely setup.
As far as the finish do like what was mentioned before and use regular bondo filler to smooth out the finishing surface. That's how professionals usually use it. I remember that's also how custom car shops build custom door panels. (Stretch Cloth over panel skeleton - Paint w/ Resin, form with Bondo-Hair (short hair), Sand, Smooth out surface with regular Bondo, Sand till smooth, then paint)
Also know that Bondo-Hair sets up harder to sand then regular Bondo so try to start knocking the high spots off it right before it's completely setup.
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