I did my entire sail with Fray Check this weekend. The only place I really had a fraying problem was one side of the bottom edge or foot and it was only fraying on one side but I decided to do the whole sail anyway.
It took almost two bottles to do all of the cut edges on both sides of the sail. I cleaned my garage floor and laid the sail out then used a knee pad to move along the floor. I found it helpful to draw the bottle towards me along the seam. The liquid is pretty thin, wicks easily into the material and dries quickly. It took a couple of hours to do the whole sail and I was glad I used the knee pad by the time I was finished.
When I was almost finished my wife the sewing\weaving expert came out and informed me that the zig-zag pattern the sail was sewn with made it entirely unnecessary to do what I was doing.
Chris