I was starting to get up a head of steam and ready to fire off a lengthy post until I read this one. I agree with Capeman 100%!cape man wrote: ↑Mon Mar 29, 2021 2:17 pm Not to add another answer to what was a simple question... but I prefer wood flour mixed with epoxy to the consistency of thick peanut butter for my filets. No sanding required if you apply the fiberglass within 24 hours of the filet. Wood flour and epoxy IS very strong, and perhaps the cheapest filler you can find (it is the cheapest sold here). For structural components like filets and overfilling holes that will then hold a screw, wood flour is really great stuff. It is harder to sand than some of the other fillers that are more suitable for fairing compounds.
95% of the fillets in my boat are a mix of epoxy/woodflour, and I am in no doubt that they provide significant structural strength as well as provide a smooth path and workable radius for glass tape.
The other 5% have some carbosil in the mix for vertical fillets that otherwise might sag.
Each to their own, but personally I can't imagine adding micro-balloons to structural fillets!