by Pete W, 2017-09
Let's try 3M Super 77 spray adhesive, and a primer / cleaning procedure. This is going to be a bit of a skill, just like board repair, but not as bad I think.
The monofilm is something along the lines of Mylar, which is a polyester (PET). Similar to the stuff clear plastic coke-bottle's are made from (PET-E), but with even MORE additives for elasticity. Anyway, PET reportedly glues well, it just needs some cleaning. Heptane  is recommended in industrial catalogs as a primer for PET -- e.g. if the surface has been weathered and/or contaminated, as will be the case with all sails. Diluted Acetone may be substituted by the outback practitioner, but really, is a bad idea if not diluted. A Light touch of very fine grit sandpaper (followed by cleaning/degreasing) can also be used as a "pre-pre-" step.
Dacron fiber is also PET, but in actual application, the Dacron weave is coated. The coating material is also called "dope". Some sail constructions are laminate sandwiches with Dacron being just one of the layers, and again the key point is there is another material applied during the lamination and finishing process, so that that outer surface is something other than PET. This is supposedly why it won't glue well. Such Dacron is supposedly even used as easily peelable backing during construction processes, specifically because it won't stick. The Dacron dope / coatings vary, and are proprietary mystery meat with folklore and brand loyalty among sailmakers. Similar to West System for boatbuilders. Two of the more popular possibilities are polyurethane (PUR) based coatings (imagine the smooth waxpaper-y hand-feel of a new plain sailboat sail... that's PUR I think.),. Another popular formulation is supposedly made from butyrate.
The thing to do is find an adhesive compatible with both the PET and PUR or whatever coating the dacron has, OR, a primer which will activate the PUR/whatever surface. 3M super 77 (adhesive for automotive interiors) is reported to work for both PET and PUR, so I'd like to try that with a patch of scrap sail material. I think trying some sanding and diluted solvent primers (heptane, acetone, MEK) on the Dacron is another avenue. That'll be a bit of a science project, if anyone is up for it.