: Structural fillets are best made with fumed silica. Silica does not sand
: easily fillets they must be formed with tools (credit cards or tongue
: depressors). The West System Users Manual is a good read, the section on
: filler selection is linkd below.

I've been using microfibers for structural filets, is just as strong as those with silica but is a little lighter, also a little more tricky to get a smooth mixture.

You can also make structural filets with a much bigger radius with less strong fillers like glass bubbles etc. But you have to increase the radius about 2 or 3:1 to compensate for the filler's reduced strength, so the mixture ends up weighing more as much more epoxy is used.

Bill H.