transom repair question

[WillyC]

I have a 1976 20' sf that I'm currently rebuilding. I've removed the plywood core from the transom (from the inside) and started grinding/sanding the last thin layer of plywood from the transom. There appears to be a layer of fiberglass/mat bonded to the gelcoat. My question is how much of this fiberglass layer do I need to remove by sanding with the grinder. There are areas where this fiberglass layer was not bonded to the gelcoat and water was actually trapped in between. I sanded away the areas that were not bonded well and don't know if I need to completely remove this fiberglass layer or just sand the surface down for a good bond with the new epoxy. Also what grit sanding disks will give a good bond with the epoxy. I started with 24 grit and dont know if I should hit it with 36 or 50 grit next. Thanks -Will