As others have said: "with fiberglass, nothing is permanent" With aluminum, drill a hole wrong and it stays wrong. With fiberglass, just fill and try again! It is a very flexible, if not messy, medium. A few boxes of rubber gloves goes a long way.
A good quality scale is very helpful. West System epoxy mixes 5.36:1 resin:hardener by weight. I picked up a 0.1 g digital scale from the aviation section of Amazon, and now measure all my epoxy by weight (tho, the West System pumps do a good job as long as they don't get an air bubble). The stuff sets up perfectly every time this way.
N430WB RV-7 #74152, tip-up. Working the baffles
Log at: http://rv7.wbencze.com
Defeating gravity one rivet at a time
VAF 2019 donation happily made