Hey rookie,

I'm assuming you're talking about the outside of a kayak after the inside's been filleted. In that case, either would work since the internal fillet and tape will provide the structural strength for the joint. The filler is just to fill any minor gaps so that the hull will be smooth enough to glass.

Woodflour will be stronger. It will also match the wood color better, which is important if you're finishing bright. If you're painting, that's not an issue. It'll be a bit more work to sand than the microballoons.

Best bet is to go with what the manual says.

Laszlo

Thanks!

I used a mix of woodflour and cell-o-fill.  1 part wood flour and 2 parts cell-o-fill to keep the color lighter.  It looks about the right color.  You might do a couple small test batches on scrap wood.