Yesterday I ground the gelcoat off the top of the cabin top with an 8 inch pad sander and 40 grit. I am always surprised by the thickness of the gelcoat on Pylatesteki. She has one full coat of cream colored gel over a full thickness coat of black gel, and it is hard as nails. (though cracked and crazed like a broken egg shell...)
I have decided to leave the hatch rails in place for the moment and core the outboard sides of the top, before the rails come off. I am not sure how much influence the rails have in holding the top straight and true. There is a chance that removing them without adding back something else will warp some lumps and bumps into the top.
I ground 1 1/2 inches around the new foam, and set the foam 2 inches away from the edge of the cabin top on all sides.
This way I can bevel the foams edge, and laminate new fiberglass on to the top without grinding the sides of the cabin, which requires grinding a recess greater than the thickness of the two layers of cloth that will be going back in order to fair back smooth.
I'm going to build up the perimeter around the foam with strips of scrap fiberglass to within an 1/8th inch of the template I made of the existing corners. I'll then use some epoxy putty fairing to pull the edge into a smooth slick surface in 2 or 3 coats, using the template as my spreader. This is faster than shaping the fiberglass its self into the perfect shape with a grinder or 36 grit, with less damage to the cabin sides.
The solid fiberglass edging will keep stray anchors and heavy stuff toted on deck from crushing the top skin over the foam, which otherwise doesn't take point loading very well.
I ran out of bricks (I thought 14 would be enough...) to hold the foam down to the cabin top. I normally use a heat gun to relax the foam a bit, but forgot to bring it along.
I used some of the pre-existing holes for winches and turning blocks to jab screws into the foam to lock it in place. My drill was with my heat gun... It works fairly well to run a sheet rock screw through the foam into the glass behind it when gravity doesn't want to co-operate.
I left the screw holes open, even though some glue will drool through I did not want to bond the inner skin to the outer skin, as I'll be removing the old coring and inner skin and old balsa for more head room.