I agree -- why do you need NEW holes? Did the old ones wear out?
We decided we didn't need our 32's depth through-hull any more; it was leaking for some reason and by all accounts it is quite simple to install an internal liquid mount in PVC pipe which is as effective.
So after popping out the old through-hull, using a $50 Black and Decker grinder, a $2 coarse grinding disc from Home Depot (the thick sandpaper style, not the black stonelike material), my boat partner ground out a 6" diameter bevel around the old through-hull on the outside of the hull, and something similar with smaller diameter inside the hull. (This took him well under an hour, less time than it took me to do some patching to the rudder!) We epoxied increasingly large circles of fiberglass cloth onto both sides. Probably 10 layers total. After fairing and painting with antifouling, it is indistinguishable from the rest of the hull and DEFINITELY WON'T LEAK.
I considered this is a very easy repair with a grinder. Much more difficult with just a sander, no matter how coarse the sandpaper is. The only complication was getting the epoxy to stay in place and not sag when laying the fabric layers upside-down on the outside hole.
See Don Casey's Boat Repair book for more details. There aren't many more.
I am increasingly inclined to do this with the speed throughhull next year. For what we do with the boat, the tiny incremental value of knowing speed through the water when you already have GPS speed over ground is not worth the PITA associated with cleaning the barnacles off the speed wheel, the instrument's inaccuracy, drag, and the small but nonzero risk of the throughhull.
