A few things-
That fin is likely solid glass/resin, not wood or plastic.
From up to about 1/3-1/2 up the fin to the base, I would sand away the glass that was added to hold the fin on the board, not a helluva lot more.You want to avoid a lot of glass buildup there, tends to be lumpy and look like hell. Try not to sand into the fin.
Likewise, my method is to sand the old rope and glass on the bottom flat but don’t sand away the ‘football patch’ on the bottom. It’s there to reinforce the (typically) single lamination on the bottom versus the torques and pressures the fin exerts in use and abuse. You don’t want to weaken that, nor do you want to risk compromising the bottom lamination if you over-sand. Sand enough so that the new glass gets a good grip but beyond that, not so much. Adding another football-shaped patch wouldn’t be the worst idea. Strength is good.
Okay, what next? Get a hot glue gun. You want to mount the fin on the bottom solidly enough that you won’t cant it over or skew it while glassing. Some pencil lines and cardboard jigs to make sure it’s lined up right are your friend here. Pencil lines, not pen or sharpie or whatever. Once the fin is in place you can wipe them off. Reinforcing the hot glue a little before glassing by running masking tape from the end of the fin to the rails, it might be useful, it might be psychological, either way, not a bad thing.
No, five minute epoxy won’t work, besides which it turns really ugly in the sun. Hot glue is lovely stuff, it hardens quick enough that you can hold the fin in position for a minute or two and you’re fine. Just a thin bead to set the fin on, don’t slather it on, You’re not bedding the thing and any that comes out the sides you will have to remove.
Awright, now we get to the glassing. I would use laminating resin, you are gonna be doing several layers and doing them all at once can go bad for somebody who hasn’t done a lot of them. Or else plan on sanding between layers. Get some thin plastic gloves, this is a job where you wind up, say, holding one end of the rope or cloth with a fingertip as you brush resin into the other end. I like a 1" throwaway ‘chip’ brush for this , nothing larger, you will want several for glassing and hotcoat and gloss. Your glass rope, cut it just a smidge longer than the fin base, let it go out a little beyond to help blend the fin base and the bottom.
Cute trick, cut the bristles on the brush you use for glassing shorter, that way they almost squeegee out excess resin in the cloth…Dont push too hard, that can get air bubbles in there. Shoot for ‘smooth’.
You mentioned using three layers of 4 oz cloth, myself I think that’s kinda light, but if 4 oz is all you can get hold of, use more layers. The three layers of 4 oz may be fine for a thruster fin, this is bigger, more stress on it. I tend to go with the first band/layer at about 2" wide, then bigger and bigger, good overlap past the layer under it. Cut your cloth carefully and brush gently, you don’t want loose fibers getting pulled out of the cloth to make sanding a misery. Onwards and onwards. If some goes past the front and back of the fin, that’s okay, you can cut that excess with a razor knife.
If you used laminating resin, I’d give it a light hotcoat to seal it and let it be for a bit until you can sand it/feather it. If you used sanding resin, you have been sanding lightly between any layers that were allowed to harden… Then hotcoat, to fill wee dips and such, sand smooth, clean dust out well and then a bit of gloss. Sand if need be, wet sand, polish, have a beer.
hope that’s of use
doc…
