Hi if the sail material is a flat surface like mylar film double sided tape is great. If it is a woven surface then there is a product called Wunda Web which is a loose filament tape which is glue and it can be laid inside a seam.
This is then ironed with the heat on medium to activate the glue to the two surfaces. Not sure if this would suit nylon or rip stock but would be good for a light cotton material.
I have sails made in this way over 10 years ago with no sign of deterioration.
If you have a material which when cut has threads ready to fall away then using a cotton bud stick apply a light varnish edge on to the cut adge and this will seal it form fraying.