Wet sanding is done with sand paper made for sanding wet. It a very fine grit 220 or finer. To do it you plop your sandpaper in a bucket of water and then begin sanding the kayak. My guess is you normally would not go to the wet sanding stage until you start varnishing the kayak or maybe right before you were going to varnish. The advantage of wet sanding is that the sand paper doesn't clog up and you also get a much smoother finish. You put the sandpaper back in the bucket of water and wash off the accumulated paste that you picked up from sanding. You need to wash the boat and dry it once you finish wet sanding it, which is the disadvantage of using it versus dry sanding.