stiffen sails - how often?
Ah. You're referring to what Sailcare does.
We've had two dacron sails refurbished by them and they come out stiff and clean. The process strips off the impregnating resin, cleans the sail and re-resins the sail. BUT the process will NOT change the shape of the sail or eliminate any stretch that's occurred. And it only works for Dacron. The process DOES stabilize a sail in whatever shape it's in. So for that purpose, it's good, but you're not getting a new sail.
Bottom line: if a sail is out of shape and a sailmaker can't recut it, get a new sail. If it still has shape, hose it off, dry, and fold for the winter. Stripping resin from a sail that has good shape is not a good idea. I would not use the process again, I'd get a new sail.
Sabre 38 "Victoria"