if i already put the deck on...and i was trying to be sensitive to minimizing the amount of epoxy, i would predrill a 3/4 inch toggle hole into the hull.

then i would place clear packing tape over one of the holes on one side of the kayak.  

i would then place the kayak on end at the angle that would minimize the size of the triangle required to cover the hole.

i would then create a woodfiller thickened epoxy with the consistency of a thick milkshake/yogurt consistency and use a little spatula to trowel it into/force it into the open hole until i saw that i had covered the hole from both sides (remember the other side has clear plastic tape over it so you can see you covered it). then tape the open side to hold it in place while it cures.

once cured, redrill the toggle hole using a 1/2 inch bit right down the middle of the original 3/4 inch hole (this will give you a 1/8 margin of epoxy around the hole.).confirm after you complete the 1/2 inch hole that the complete hole has no opening into the hull.  then you are done.

on the question of how far back from the tip of stem/stern are the holes, i think this is sort of an artistic question....what looks good.  in my boats i tend to keep them a bit more forward then what the plans call for.  i use blue masking tape cut into shapes of circles  (3/4 inch circle) to allow me to paste the hole on the hull temporarily and adjust where i think they look good...then i mark the center with a pin....so i know where to drill.

hope this is helpful.

h

i would not bother with cell-o-fill. wood flour will also better match the wood when you drill your hole with your 1/8 inch margin.

Thanks much.  I like the idea of predrilling the holes and using them to gauge the amount of epoxy needed to cover them.  Not sure I understand the significance of the eighth inch margins.  Why not just a half-inch hole?

Hi Florida Jim. 

  you may have heard the term, "drill, fill, drill" ....but basically, you want to ensure that you don't create a way for water to get into the wood hull panel by the end grain exposed when you drill a hole..

so if you drill a 1/2 inch hole and fill, and drill it out again at 1/2 inch ....you will have exposed grain of you plywood.

you can coat the end grain with epoxy.....as an approach and that may work, but by leaving a margin around the hole....it is just a more resilient aproach to keeping water out of the end-grain of the wood.

h

Got it.  Thanks!