I use 100 grit - final hand sanding (with grain) on the outsides and 80 or less on the inside. Do a thourough job with whatever grit you end up with because the finer the grit, the lighter the wood will be. In other words, don't hand sand one section with 120, because it won't wet out as dark as the 100 grit area. Which in many ways tells you something about about the way the epoxy is being absorbed into the wood.

Epoxy is not water and has viscosity. There's no way a 220 sanded surface is going to absorb epoxy like a 100 grit sanded area. So, what's good enough. Only time will tell. All kayaks look great when first done. Will the movement of heat (expansion and contraction) and years of use - cause adhesion problems? I alway fully realize that this is boat building not furniture making. I personally trust a coarser grit because the epoxy glass coating is not just for beauty but structural as well. How much furniture sits in the 100 degree sun all day, lays in ice cold salt water and gets banged and twisted by the forces of paddling and water pressure? I highly recommend a coarser grit for kayak making. 100 grit whan hand sanded with the grain yelds a beautiful, smooth surface that creates a good bond. West System has done bond experiments and beyond a doubt, coarser is better.