Generally the stitching holes are drilled through the side of the coaming, and I think it would work fine with stacked plywood. I have tried drilling through the top, but found it to be difficult to get a needle through, although it worked fine once completed.

The Skin Boat School has a lot of good instructional videos on skinning - here's a link to the one for sewing the coaming: http://skinboats.org/skinboats/instaling_cockpit.html

Generally, the skin fabric is pulled up tight against the inside surface of the coaming, so when coated with polyurethane, it seals the the stitching holes. Now, if the stiching holes are excessively large - so large that polyurethane will not seal them - that would certainly present a problem. In that case, I would try some kind of clear sealant, such as Lexel, Aquaseal, etc. smeared into each hole.

: I'm working on my 2nd Yost boat, a Sea Otter R. I expect to do a sewn-on
: skin. My combing will be Yost-style plywood buildup with rigid ring on the
: frame, as that's what I'm comfortable at building.

: I see sew-up skins are stitched onto the combings (which float? I have a hard
: time grasping this). So, how are the stitch holes in the combing riser
: sealed? This is probably of extra importance with a plywood riser. Or,
: should I be considering a some method other than stitching to the combing?