That was it... 5200. Obviously not something you can do quickly, but I heard
it was better than tape for the long term.

how long is long term? I have seen sail repair tape/cloth hold for more than 5
years. I have used it myself on my own sails for that long without problem
(the sails plain worn out without the repair tape/cloth giving up).

5200 does work, however, for repairing inflatable dinghies. make certain you
let it set long enough before inflating.