Um, at the risk of sounding like a dumb neanderthal -- and no offense to the innovative solutions offered by others here -- I don't think fillets should be all that tough to deal with in the context of the overall project. What worked for me was simple: Mix wood flour with epoxy thick enough so that it won't run. Work NEAT. Sand off any excess. Presto! Perfect fillets.
Since the fillets are thin anyway and generally exposed, they sand very, very easily (at least compared to the entire boat after it's glassed). Plus, with careful sanding you can mold them exactly like you want.
I don't know. I've only built two boats. And sometimes on these bulletin boards, I find problems and solutions are more complex than necessary.
It's understandable, though, because I know these projects and the idea of tackling hardened epoxy can be daunting. Good luck with whatever solution you come up with. As for me, I'm hoping the sanding strengthens my arms for the upcoming paddling season.
Cheers, Grambo

: Question: I am having trouble getting smooth fillets. If the epoxy is thin
: enough to lay smooth it runs and if it is thick enough to not run it is
: rough and hard to tool smooth. Does anyone have any tricks or ideas to
: make a smooth well formed fillet. Sanding after they are hard is not much
: of a solution... I've tried it.
: Background: I am building a 17' S & G Kayak and I am having trouble
: getting the fillets smooth and well shaped. The thicken epoxy clings to
: the tools( cotton filler in East System Epoxy) or if not so thick it won't
: stay in place on the inclined sides. Any comments would be appreciated.
: Thanks