There are a lot of ways to resurface a plastic bathtub. Here is just one example:
1- Wash the tub with soap and water. Scrape off any old caulk around the base of the tub. Sand down any bumps or ridges on the surface of the tub with course-grit sandpaper, but be careful not to damage the surrounding wall tiles.
2- Clean away all dust from sanding with a damp cloth and let dry for 24 hours before applying a new surface coating.
3- Apply an epoxy primer over entire bathroom surfaces where new caulking will seal gaps between wall tiles and tub or shower surround.
4- Use a roller brush or paint pad to spread epoxy over the entire area, then let dry according to the manufacturer’s instructions.