I've searched quite a bit already, my plan so far didn't come out of thin air after all, and watched all the videos I could find on youtube. They never say exactly what glass to use for a repair, though what I found about glass types seemed to indicate that stitched biaxial would be stronger than just cloth. Also that mat has better adhesion than woven fabric, so boat hulls are usually made with alternating layers of woven roving and mat. So stitched biaxial with mat backing seemed like the right choice. I don't see anyone selling 7oz fabric, mostly just 4, 6 and 10. I take it four layers of 6 oz fabric should be about as thick as one layer of 1708?
West's boat repair manual
, section 2.2, says "these finishes are recommended for above the waterline only. Use west system for all repairs and coatings below the waterline, prior to applying bottom paint." It's not entirely clear to me if the warning about below the waterline use applies only to polyurethane paints or if it includes polyester gel coat as well. The section on repairing keels and rudders says to apply several coats of epoxy after fairing, then wet sand, then bottom paint. No gel coat or barrier coat.
Evercoat's gel coat information
says quite clearly on the first page, "IMPORTANT: GEL COAT SHOULD NOT BE APPLIED OVER AN EPOXY."
So the gel coat maker says you can't put gel coat over epoxy and the epoxy maker says you can (but maybe not below the water line). That really clears things up. My searching found a lot more people saying not to put gel coat over epoxy than the other way around.
This west video
says after fairing, coat with two coats of epoxy with a white pigment added. Of course it doesn't go into any detail about what happens after you're done using west products. Maybe you go direct to bottom paint or maybe there is something else? This west video
just says to paint or apply gel coat after the last glass layer is dry. No fairing even. This other west video
says to finish with six coats of epoxy combined with 422 barrier coating additive (aluminum & mica powder), then bottom paint. I'm pretty sure I've seen videos involving gel coat after fairing too.
I haven't see any guides describe patching from both sides. It's always just one, though I have found discussions about which side is better. Any links to a patch from both side guide? I've looked at tons of them and don't recall any, yet now two people suggest it.
Both sides has an appeal, as half the hull thickness to taper will give a patch with half the radius, which is just one quarter the area. Two 1/4 area patches is still just half the total patch area vs one side patching. The problem is that any grinding on the outside of the boat is very costly. Full tent, vacuum sander rental, etc. Hundreds of dollars. Assuming the boatyard will even allow it in the first place. Plus I have a bigger spot on the outside to fair and paint.