Unless you wish to greatly diminish the sail cloth's bias
strength

Had a nice conversation with Steve Pearson, head designer at Wills
Wing, about the many aspects of sail cloth. As an aside he mentioned that using
AP 303 (http://www.amazon.com/303-Products-AQ303AP-Protectant-oz/dp/B004MAOEKG#)
on your hang glider sail, although praised in many on-line forums, was a
terrible thing to do because it diminished the bias strength of your sail
material. It improved the tear strength and softened the cloth (better handling
perhaps) but was a really bad idea.

He writes:

The bias strength could be diminished by 50% or more but the
strength in the orthogonal directions will be minimally affected. Essentially,
you are severely compromising the unique structural properties that distinguish
sailcloth from conventional fabric.