Quick and Safe Sail Cleaning
Oxiclean and dirt cheap DIY formula match specialty cleaners.
It can be a rust stain caused by a loop of chain that spent the winter lying on a sail. Perhaps a bird crawled under the sail cover and built a nest, pooping on the sail for weeks. Laminate sails present a particularly vexing problems, since mildew likes the adhesive that bond the layers, resulting in stain that is sealed between waterproof layers. Some of these just look bad (rust stains), but others can slowly weaken a sail (mildew in the laminate adhesive). But in all cases, overly aggressive cleaning can make things even worse, weakening the sail more than the mildew ever would.