I have yet to varnish a kayak (maybe this summer), but I have refinished my share of furniture. I always thin the first coat 50% and every coat thereafter 25%. I've found that thinning the varnish helps eliminates brush marks and drips, but it does require more coats with light sanding in between compared to applying it right out of the can. I have always believed that the thinning medium simply evaporates away and does not affect the performance of the varnish. I hope some experienced kayak builders weigh in on this issue. /David