I have used "Wink" rust remover and it works pretty well...Don't know the long term effects as far a weakening the cloth...it is an acid.
Try it on obscure portion of the sail if you want to test it...flush it with plenty of water after the stain has vanished...it is made to be use on clothes (whites and colors), and I use it for that all the time, so it should be fine... I used it on an old white tornado sail once, it worked amazing...the sail had some really nasty rust stains and it made a significant improvement...most times it completely removes it instantly without a trace of the original stain. If its really bad...to the point of having a "crust" of rusty metal on the fabric you may have to keep apply over and over till it dissolves it all.
You can buy it at most grocery stores, very inexpensive...Publix has it in my area...comes in a small dark bronze plastic bottle. If you look at the label, it probably has the same active ingredients that the high dollar "marine" version has...
PLEASE NOTE: This recommendation comes with no warrantee or guarantees, expressed or implied...LOL
Regards,
Bob