I'm a big fan of DuPont Supplex. It's strong, light, UV resistant, and being a synthetic cuts with a hotknife which seals the edges. Holes for reef points, etc, I make with a soldering iron point.
Here's some Supplex sails on large working models:
Dan's
Syren, a super-detailed SC&H Cruiser class brig in 1:24 scale. SC&H supplies Supplex sails for it's kits - Dan just made them more accurate with detailing.
A closer look at
Syren - "the clothes make the man" Dans been sailing with these sails for nearly five years now with no issues.
Victor Yancovitch's
Royal William in 1:36 scale.
Royal William My sloop of war
Constellation in 1:36 scale.
Constellation (video capture)
Just made mains'l on my
Pride of Baltimore in 1:20 scale.
Some more details of making
Constellation's sails on my web-site.
and
Pride of Baltimore's as well.
The only sewing I've done on my sails are the bolt-ropes. Seams are drawn on and patches, tabling, cloths, etc are all glued on.