Get both. Wood flour is fairly strong and fairly hard to sand. Silica is very strong and very hard to sand. Wood flour is strong enough for most applications, but it drags when mixed thick enough to make fillets and it sags on vertical surfaces. Silica is stronger, but if the plywood will break before a wood flour fillet, how strong does a fillet have to be on an average 12-20 foot inshore boat?
Silica does have a place in your arsenal. It makes a very smooth fillet that doesn't drag or sag. Wood flour seems to take less to initially thicken the epoxy, but I can't seem to get a wood flour only blend thick enough for fillets without ending up with a mix that acts like sticky biscuit dough. I use wood flour as cheap bulk to get my epoxy to a molasses consistency and then use silica to get it to a peanut butter consistency. My fillets started to look better immediately and I didn't have to sand as much to get rid of the sags. A word of caution: once you mix your fillet blend thick enough it won't sag, it cooks off faster, so work fast. The flow of thinner mixes allows the mix to dump heat, but once you stop that flow, all that heat just stays in the mix. It will have 5 minutes of pot life, especially when you are working with fast hardener in the winter time. If a bead of silica/wood flour epoxy cooks off before you get the next piece bedded into it, you'll want a side grinder with a 30 grit disk to get rid of it. It eats sander belts.
On my 16' hull glassed in side and out with 6 oz cloth with bulkheads, decks and full gunnels, I've used probably 12 qts of wood flour and I am on my second 5 qt container of silica a this point. If I had used silica from the beginning, I probably would use 15 qts of silica and 7-8 qts wood flour. I've also used 3 qts of microballoons, 3 qts of Quickfair and about 11 gallons of epoxy/hardener. If you make a hull with only bench seats and you don't fair it, you will use half of what I did.
Nate
