In the past I have used Oxalic Acid (Wood Bleach at Home Depot). Make sure you dilute it well. Just leave it on, every once in a while you can agitate it with a soft brush or sponge to see how it is coming along. Then rinse off when you are happy with results.

I have Pentex Sails now so I have not done this since I owned old dacrons.

[image]http://www.catsailor.com/gallery/albums/userpics/10093/CatsailorSignatureGIF.gif[/image]