Penetrol is a paint additive, you can pick it up at any hardware store for about $10/can. It has a side use as a fiberglass restorative for boats--or Jeep hard tops. It takes a few days to dry and it's sticky when first applied, so do it in a garage and leave it alone or you'll get bugs and dust stuck on. Just paint it on with a sponge brush, I use less than 1/2 can for the hard top every few years. I mean, it's amazing the job it'll do. Your hard top will look freshly painted and shiny. I just put it on my 16 year old hard top a week ago, I'll get a pic up this afternoon. But google penetrol and Jeep hard top and you'll see some examples.
I LOVE Monstaliner in my tub. But I really think it's designed for hard surfaces like metals. Fiberglass can flex a bit and you don't want it to flake off.