Another thread here is Fear of Fiberglassing -- my husband went from not knowing a thing about it to being able to fix everything from a 2 x 3 foot hole to long cracks above the door, to missing sections where it had broken away...
waxed paper over cardboard can be used to "back" a hole; you can thread a cord through it, fold the cardboard and shove it through the hole, then let it flatten back there, pull the cord ends out onto the front side and tape them securely with blue masking tape, and later trim the cord off before finish work.
We use epoxy resin; many prefer polyester. Epoxy is stronger and sticks to almost everything (not waxed paper) but costs more. Polyester doesn't stick to epoxy BUT to polyester and is more flexible and less expensive.
OUR rig had so many different materials and strange things that we started with epoxy and never found a reason to change.
There are strong opinions out there about it...if you do enough research you'll find what you'd like to try.
Good luck to you...once you start working with fiberglass, you'll be glad you did! And Bondo Hair (the fiberglass version of Bondo) and similar products are just great for smaller holes, narrow cracks, etc.
Remember, don't skip steps. Do plenty of careful prep and follow-up and you should be fine. Any paint
should be rated for use on fiberglass -- we like Rustoleum Marine for wood and fiberglass, ourselves. They make a nice primer (we use the white colored primer) as well as good paint
in a few colors such as white, blue, etc.. But what you choose depends on what's available for you, locally or online.
And the fiberglass products aren't really cheap...but they won't ruin you, either. Probably. 8) =)