The boundary layer is more of a film below the water line . I would guess this layer would weigh only a couple of ozs. Nick hinted in the previous thread that the benefits of a sanded finish would be debatable . From what I can gather from reading a harder finish than the spar varnish I use with a fine sanding below the waterline might work best. I like the idea that i can drag my kayak out of the shore break by the bow loop abrading the keel stern area and recoat with spar varnish at end of the season. Other 2 part spray finish products would be more labor intensive. Having said that in the evenings where I paddle a group of serious surfski paddlers meet up and their hulls never touch the sand. I think they really do not work on their surf skis.So keeping a gel coat intact is imperative for structural and cosmetic reasons. I have been thinking along these lines because a N.Shade Mystery kayak will be next build as soon as i finish my second petrel for my wife. The bottom line is tune the motor. In the open ocean it would also help to quit zig zagging and sanding a harder finial finish will not fix that .