From a sidebar entitled Sailcloth Care on page 44 of the 2006 catalog:

The following methods (detailed in Sail Repair Manual) can be used to remove or reduce spots and stains:

RUST--soak the affected area in a 5% solution of oxalic acid dissolved in hot water. Follow this with a 2% solution of hydrochloric acid in warm water and , finally, rinse well with clear water.