I used West Marine's structural fiberglass filler on mine. It contains chopped FG strands. It is very strong, it does not part from the gaps you work it into, and it sands very well, so I have no complaints. West System epoxy is fine as well but will be much harder to sand using conventional methods. I do not know why anyone would NOT use the 3M 'blue' filler on lead-- that's been one of its primary applications in the production world for as long as that stuff's been on the market. I used the WM brand only because it's what I had and it was cheaper.