I've used it on both polyester and epoxy filled fiberglass, and as long as you give it some "tooth" to grab onto and get rid of any surface wax etc. it will hold like grim death. If you use it on a finished surface though, remove as much excess as you can before it sets up. JB Weld cures fairly hard, so any sanding may remove the wrong material first. I've seen this when using it as filler on a styrene hull, for example, where the hull is relatively soft. Also, JB Weld tends to be a bit brittle, so if you use it to span a joint where there might be some flexing you may want to add some filler material, like chopped glass, to prevent cracking. If you are just laminating two flat surfaces, however, this wouldn't be a problem. I would give it a good 24 hour cure before stressing the joint. Having said all that, its one of my favorite bonding materials and, as Pete said, its quite waterproof.
George "Crazy Ivan" Protchenko
“There are the assassins, the dealers in death; I am the Avenger!”-Nemo
"I'm disinclined to acquiesce to your request; means No!"-Capt.Barbossa