Hey
@fred1diver
I have done a lot of fiberglass work over the years. Here are my steps for filling a hole (b.t.w. I'd also recommend this for the through hull valve you showed us for your head on a different thread).
1. First grind the edges of the hole into a flared disk around the hole. Do this on the INSIDE of your boat. In the pic below, the yellow is the bulkhead, the black is your hole and the grey is your ground out flare.
2. Next tape some sort of backing plate on the OUTSIDE of the hole. I like to use those plastic sheets that your teachers used to use for overhead projectors because it leaves a very nice smooth finish when it is removed later.
3. Cut a series of circular pieces of fiberglass with increasing diameters. The smallest one should be the size of the hole. Each successive circle slightly larger in diameter. Keep cutting rings until the stack will lay flush in the hole and come even with the original bulkhead. Step 3 is the tricky part because cutting the correct number of rings is a bit of trial and error and if you don't get it right your hole will look crappy.
4. Wet all of the circles with epoxy resin and lay them in a stack on a second sheet of overhead projector film so that the smallest circle will be on top. As you stack the circles use a resin squeegee push out the excess resin from the cloth and move the resin off of the plastic sheet.
5. Push the set of circles into the hole and then tape the plastic sheet in place until the resit sets.
6. Wait 24 hours and remove the two plastic sheets. You will now need to do a little bit of light sanding and painting but your hole is now filled and stronger than it was before the hole was cut.