I do a good bit of fiberglass work myself because I'm really into performance jet skis. It never fails that I'll get a ding or chip in the fiberglass every now and then.
I have spoke with numerous body shops in my area over the years and everyone recommends "Lord Fusor" products. I used fiberglass resin & cloth and do it the old fashion way until I started using the "Fusor" products.
This stuff is the best, hands down. It is 100 times easier to work with than old fashion resin/cloth and is probably 100 times stronger too. I've never had this stuff crack or chip.
Just attach a special mixing tip to the cartridge, use a standard caulking gun "you do have to buy a small $15 attachment but it is a one time purchase that should last forever", insert the cartridge into the attachment, and then the attachment into the caulking gun.
The "Fusor" products are roughly $12-$15 for a small tube, but it will do an amazing amount of area for that little tube. Plus, the first time that you see how easy it is to work with, you will never go back to the old way again!! It is well worth the money. This is truly, you get what you pay for situation.
Here is a picture of the Fusor "101EZ" compound that is special designed just for Fiberglass.
By the way, you can get this stuff at most all autobody paint suppliers. If you are having difficulty finding it, I'd be glad to get it for you. I'll do it strictly at cost. The store is only a block from my house. I'd be happy just to help ya out. If you need some, just PM me. Sincerely, Scott