This is an easy procedure for repairing most cracks in the shell. Small, hairline cracks can be fixed from the water side of the spa. Longer, deeper cracks must be repaired from both sides of the spa shell.
We drew a 2” line on a spa shell to represent a crack.
Using a 1/16” bit, drill a pilot hole at both ends of the crack. This will prevent the crack from spreading.
Using a utility knife, cut a v-channel from pilot-hole to pilot-hole. This becomes our channel for holding the epoxy.
Apply a bead of epoxy over the V-Channel.
Wearing a rubber glove, spread the epoxy into the VChannel. After 30 minutes, the epoxy can be lightly sanded for a finished look.
If the crack goes through the shell, or is at a thin spot near the floor of the spa, you may have to apply a heavier coat of epoxy on the underside of the shell
Apply a much larger pattern of epoxy on the back of the shell. This pattern is about 4” square.
Wearing a rubber glove, Spread the epoxy into a 4.5” square. You should have a thick layer, as if you were making a peanut butter sandwich.
Lay a 5” square piece of fiberglass matting, (or even a piece of old T-Shirt or jeans).
Apply another layer of epoxy on the material.
Spread this layer about 5.5” square, feathering the edges. You now should have a double layer of epoxy to stiffen and strengthen the bottom of the spa shell.