How can I repair a surface chip in a bathtub?
IMHO, IME, you're going to have to replace the tub to get a truly acceptable fix.
Your description of the material suggests acrylic over fiberglass, or at least that's the most common sort of &quot;plastic&quot; tub these days.
There are various products that can be used to attempt repair. They are probably better than doing nothing, but the odds of getting a repair that looks &quot;like it never happened&quot; are, in my experience, very low.
The primary difficulties are precise color matching and getting the finish of the repair to match &amp; blend with the finish of the rest of the tub. Bonding can also be an issue, but that's generally less of a problem (assuming reasonable attention to detail.)