I have no clue why your fiberglass tub/shower would be "peeling", and I
have even less idea as to how to fix something like that.
But, believe it or not, the single biggest thing you need to do when
painting a bathroom is to buy the RIGHT paint. That's because not all
latex paints are equally resistant to moisture and humidity, and low
priced paints will crack and peel under conditions of high humidity.
The advantage in buying a paint specifically made to be used in
1. You're getting a binder resin that was chosen because of it's
excellent resistance to moisture and humidity, so the bathroom will stay
looking "newly painted" for a long time, and
2. Bathroom paints will have a powdered mildewcide added to them. This
mildewcide is so highly soluble in water that even the high humidity on
one side of the paint will cause the mildewcide to migrate through the
paint film to the surface, where it will kill any mildew spores before
they have a chance to grow.
I know of two paints specifically made for bathrooms. The first is
Zinsser's "PermaWhite" available at Home Depot, and the second is simply
called "Bath Paint" and is available at Sherwin Williams paint stores.
I've been having a lot of problems with bubbles forming in Zinsser's
Permawhite lately. I literally have to use an electric leaf blower to
break the bubbles that form in that paint before it dries. So, in this
post I'm gonna warn people away from Zinsser's and suggest you buy some
Sherwin Williams Bath Paint.