Re: How difficult is this repair??
I am no expert, but I have done some fiberglass repairs. I think it looks like the underlying glass is rotten. If so, then Marine Tex will not work. If you can get to the inside area, the repair will be easier and neater.
To do it right you must grind away the bad, rotten, loose glass until you get to solid glass. Then use cardboard, waxed or covered with waxed paper, shaped to the outside of the hull and taped in place with a strong tape. This will be the form, the wax will help the glass release, and the smoother the surface of the "form" the better the result.
Working from the inside, begin by laying a piece of resin soaked chopped mat into the cardboard form, from edge to edge of the old glass, maintaining the required shape. Add successive layers of chopped mat and woven roven, each layer slightly larger than the previous layer. Work out the air bubbles and keep the layers smooth, no more than 3 to 4 layers at a time. Too many layers will generate to much heat and cure too fast. Don't use too much resin, nor too little.
Once the glass has set, remove the form, sand the surface and paint, or gelcoat as desired.
You will need 24, or 36 grit sandpaper to work the fiberglass, 50 and greater for the finishing process. Use high build primer to help cover scratches, and filler for a smooth finished surface.
This is a simplified version of what must be done, but if you do it right, it will out last the rest of the boat.
