I know how to built panel with fiberglass, using the technique
involving cloth and resin...But what about those who use the factory
plastic trim panel to make it looks glossy painted. Do you
have to put fiberglass matt and resin over it, or if you only sand the
plastic and put body filler and then paint it with a clear coat
finish...? Because I've seen many car interior that are made of
fiberglass, and panels still keep the same shape and thickness than
the factory plastic panel. So, is it necessary to make fiberglass over
plastic panel in order to get a custom paint look?

thoses smooth paint jobs are not by using fiberglass matt they are
just smoothed with filler and then painted