Here is a link to a "Good Old Boat" article / info:
http://sailnet.com/forums/good-old-boat ... -hole.html
I filled in several hull valve through holes 10 - 15 years ago. Although I do not remember the details, I can tell you my general approach. I ground the hole (from the outside and inside) into a dished out area. Then I laid multiple layers of West System epoxy soaked cloth (sorry, do not remember the weight) from inside and outside. Started with small diameter circles, working to larger diameter layers. My thought was to make a "sandwich" patch from both sides so that it "clamped" the hull to provide some mechanical strength so the completed plug would not "punch out". Next step was to keep layering from both sides until I reached hull thickness. The outside was finally smoothed and faired with additional epoxy / filler to provide a smooth finish.
The inside of the hull in the area of repair was in a difficult to reach location without any real working room around it. It was easy enough to get the patches in place, but not a lot of room for "finishing smooth." The inside got a rough smoothing (as best as I could under the circumstances), then for appearance and (maybe some additional strength) I cut a circular patch of plywood about the thickness as the original backing plate for the valve and applied it (epoxy plus filler) over the repair area inside the hull. As a final step this plywood patch got a fillet around the edge for aesthetics and easy of cleaning, plus it provided a more finished appearance.
Since the outside area was underwater and in a area where the hull had been barrier coated, I barrier coated then bottom-painted the area as a final step.
