If you over tighten the valve, the ring can "bunch up" causing it to leak slowly. After tightening, turn it looser just a hair to prevent bunching. Also whern inflating don't tighten the valve all the way until the kite is completely inflated, then tighten (that way you don't twist the bladder).
On the few low density EPS boards I've made, I used three layers of 6 oz. surfboard fiberglass on the deck. In between the second and third layer I cut 8" dia. circles of fiberglass cloth and overlap them around 50% in the area where you stand on the board. A bit heavier, but helps prevent heel dents.
Been thinking about building a low tech foilboard using 2" EPS with a cross hatch wood (probably poplar) stringer as below. Simple 6 oz. fiberglass epoxy layup with a wood veneer bottom and a top pad. Any thoughts on this?
I just performed the long mixer test on my 12m Soul. On both sides of the kite B & C were about .5 cm short. I did not adjust the mixer as I thought that would be in the margin of error for my measurements and would not make that much of a difference in how the kite flies. I suppose that if it were ...