This is a demo of that method (just using 5200 instead of contact cement):
How to Patch a Sail
It looks permanent in both the good and bad. It doesn't peel off later like sail tape, which allows it to last a long time. However it also doesn't allow a sewn repair
to be done in the future.
I disagree with this from Mark statement though: "It needs replacing when it shreds."
If you just sail downwind maybe that is true, but for upwind sailing I don't think there is anything that makes a bigger difference then going from a blown out old sail to a new crisp one. I wouldn't wait for sails to shred to call them dead, I'd wait until you can no longer control their shape effectively.