I find on my fiberglass work the auto bondo stuff to heavy for me, I use Interlux Epoxy Filler or a marine fairing compound, the product is lighter and is very sand-able in the 1st 24 hours setting harder later. I can sand and fill star cracks with it well, for other damage I go with an under coat of Interlux Epoxy PimeKote which is a two part epoxy primer, many do not like dealing with it because its messy, but as an under coater for the simpler one part product like Interlux BrightSide it works well, this is a great product for older hull/trailers.
I purchase from Jamestown distributors...also they have on-line how too videos on the products they carry.
Raya on this board is a great person to talk to on this subject and is knowledgeable in fiberglass maintenance with many sources for them...You can pm Raya here as he is very helpful.
Cruise in "Modifications and Neat Updates" topics for others and see their work often featuring the products used...I use a 2 part system because I am used to working with it, the one part epoxies have had great reviews too and are easier to apply.
I go for the UV stabilized products
as plastics and sun do not mix well...others will weigh in for you too.
Happy Camping, Safe Trails.