i am fixing the tail of an old channel islands twin fin from like the 80s it has some pretty large holes out of the tail where the foam has just been all worn away but the fiberglass on the deck and bottom are still intact, kind of like a little cave. i think filling it with resin would be wasteful and heavy. i have heard of using foam plugs to fill gaps but dont really know how to use them or what type of foam to use, any help would be greatly appeciated.
When we have a large hole to fill we usually try and sink a new piece of foam in the repair to save on weight and to give it the original color of foam. We first cut out the ding area with a router. This gives you a good flat bottom to work with. Next I take a piece of foam (that I saved from shaping) and use a high speed sander/wood block (36 grit) to get it nice and flat. Take a piece of paper and lay it on the ding. Rub around the edges so it gives you the exact shape of the hole. Take this piece of paper and lay on your scrap foam piece. Trace around the outline of the ding hole and again use your high speed sander (36 grit) or wood block to shape it to the outline. A little trial and error and you should be good. I use a little lam mixed with white pigment to cement it into place. Make sure you place a weight on top so it doesn’t ride up. When cured you can sand it to match any shape needed then glass as normal.
I had a board collector bring me Hobie that had a leash cup added very poorly as an afterthought. The hole had been made with a 1-1/2" hole saw, which was much bigger than the cup. I made a router temp for a 1-9/16" hole and plugged it with foam to fit. Anything bigger than a thumbnail shouldn’t be filled with just resin, household sugar and resin makes a very easy, grainy, mixture to sand and mould to shape