Re: Removing old thru-hulls
I think you are on the right approach to remove the old valves and make the areas water tight. if that were my project I would sand a circular beveled area away from the valve/thru hull on both the inside and outside. it might be tough when working inside due to limited space but bring it down a little but not much, say quarter to half inch to get the glass off the thruhull. If you use a grinder do so with caution. Grinders remove too much to fast. I have a festool Rotex 150 sander that can take away a lot of material but provides good control. A dremel sander would also work.
NOTE: you do not want to remove all the material around the thru hull, just enough to get down below the surface. the bevel should be about 12:1. That is, if your diameter of the thru hull is about an inch create the circular bevel out about 12 inches from the center of the hole deep enough to expose the face of the thru hull (i.e., on the outside of boat). see if you can use a step wrench on the outside of the thru hull while trying to turn the gate valve. you might have to chip away paint around the steps in the thru hull. you will need a helper for that part of the job. if it doesn't move remove the gate valve, if frozen you may need some heat to break the rust, be careful do not hold heat for to long. I have used a sawzall to make three vertical cuts spaced evenly in the thru hull being careful not to cut into the fiberglass of the hull then use a small pry bar or old flat head screwdriver to pry away the pieces of the thru hull. You might have to modify the metal sawzall blade to fit your opening.
Once out clean the fiberglass with a good solvent like acetone. fill hole with a thickened epoxy. a good thicking agent would be collidial silica, then rebuild the beveled area with wetted fiberglass cloth, probably 6 oz. will work, then use a fairing compound, and and smooth areas to match existing hull shape, cover with one or more coats of barrier paint then bottom paint. Inside same procedure except barrier and bottom paint. Other good sources for this type of project are Maine Sails website and West System Epoxy/Gougen Brothers. There are other epoxies I just prefer west system.