I practically rebuilt my original ABS plastic shower pan and seat using marine grade (think West System) fiberglass. My advice is as follows:
1) Do not use the polyester based "auto repair" fiberglass you can buy at the big box stores. It won't even stick to the ABS plastic. There is some advanced epoxy based automotive stuff designed to stick to the plastics bumpers are made of (I think they are designated "SMS"), but this stuff is super expensive, and really toxic smelling. Go with Marine grade epoxy based fiberglass, and you should be good to go.
2) Apply the fiberglass to the OUTSIDE of the tub and seat. This way, you can be a messy as needed, but the surface everybody sees remains smooth.
3) To refurb the finish on the tub, I tried two different brands of "bathtub rejuvenator" type paints. Both left me less than impressed (one was made by Rustoleum, don't recall the other). I finally found that the glossy white epoxy based "Appliance" spray paint yielded the best results. Have yet to see how it holds up in the long term.