Reward for lost Kraken!
Join Date: Apr 2006
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Often tape doesn't seem to hold on sails. I know part of that is because there's usually some microfine salt spray that acts as a release powder. But something I found out (don't ask me how or why) is that if you use a good glue/epoxy and then place a good tape over it, the tape gets permanently bound to the glue.
On a sail, I'd vote for urethane glue, like Goop, applied after washing the area clean (water then alcohol), topped with sail tape or gaffers tape (which is similar to duct tape, but 3x more expensive with an adhesive that doesn't turn to goo). Plumbers Goop is supposed to have things in it making it bond better to plastics, so I'd try either that one or plain Goop.
And then of course if you can--still stitch over the repair.