epoxy fairing over gelcoat

I have a older plywood boat hull that was sheathed with chopped strand mat with gelcoat over all. The glass appears to be well attached but the surface is quite rough. The mat strands show and the gel coat was painted on. I want to get the hull bottom smooth and plan on using epoxy for fairing. I know I have to degrease and sand the surface to get a good bond. If I sand enough to get the surface smooth I will be sanding away a lot of the mat but of course that will make the fairing easier. If I don't sand in all the nooks and crannies the fairing compound won't get a good bond. Does anyone have any suggestions on how to prepare the surface for fairing compound? The boat is a 23' planing cruiser with a cabin so I will be working with the boat upright.

 

Clean the outside of the hull with a degreaser, then go over the whole surface with DA and 40 to 60 grit, with the idea of knocking down the high spots and "toothing up the sheathing. The aggressive grit will eat up most of the textured surface. At some point you'll have to decide if it's worth getting the last 1% - 2% of remaining bits off, or just move on.

I wouldn't long board the sheathing aggressively, as you'll just cut up more material, weakening the sheathing. I would use the long board to identify the low spots. A quick pass (100 - 120 grit) on a 30 - 45 degree bias in each direction will show you what you need to know. Mark and fill the low spots and start the fairing process. When satisfied with the fairness, give the whole hull a couple of coats of neat epoxy to seal the old polyester sheathing, then prep for paint.