Hi,
I had good luck spraying my fiberglas tub with Rustoleum's 2-part epoxy kit for porcelain and fiberglass. Prep was the key. I had to repair the floor, it had cracks, but after sanding the tub looks almost as good as new. I did a lot of research and surface prep makes the difference.
Supposedly you can brush it on. I used a Wagner sprayer and built up a good coat quickly. You can get it at Home Depot.
http://www.homedepot.com/p/Rust-Oleu...0519/202246270
Good luck!
-Steve