Two part process....
For WHITE DACRON only.Cautions:DO NOT put sail into a washing machine ... the mechanical action will release/break the plasticizer that is calendered-in between the fibers.Dacron is a 'dirt magnet' so put the sail only on CLEAN surfaces when cleaning/washing. ie. on a clean plastic tarp, otherwise the sail WILL pick up dirt from the surface.Best is to do this ON the boat (but not a boat that has bare teak or bare teak decks as the detergent will also extract the tannins from the teak and transfer them to the dacron.Part1;For mildew, etc. soak the sail in a detergent that is caustic based. Sodium Silicate based detergents seems to work best as they DISSOLVE the mildew and fungus cells. Soak time is important so that you allow time for the mildew/fungal cells to dissolve.On a windless, high humidity, cloudy day ....slowly raise the sail as you spray on the detergetn from a spray bottle.spread the detergent with a long handled *soft* bristle brush as you raise the sail.Immediately drop the sail to deck (cleaned) and cover with a plastic tarp (to prevent the sail from drying)Let soak about 1 hour.Reraise and spray onto the areas that did not get full detergent coverage the first time. Do the opposite side of the sail, drop, cover and let soak about 1 hour. Slowly raise the sail and gently scrub the sail with the long handled brush, both sides. Drop and soak. With a garden hose in hand, slowly raise the sail while rinsing, drop, rinse, drop rinse drop rinse, etc. until ALL detergent is out of the sail. Expect the first rinse to cause VERY dirty water as the crud and mildew cell fragments rinse out of the sail. Most of the mildew/fungus and 'general dirt' will be removed, some shadows of the mildew/fungus will remain. Part2 bleaching and 'iron' stain reductionCan be done ONLY if the part 1 was finished with a thorough rinse and ALL detergent is rinsed out. Do NOT do if detergent/soap bubbles are present in the sail fabric. Get some oxalic acid form a hardware store. Dissolve the oxalic crystals in a bucket of warm water. Add the oxalic then fill with enough water until ALL the oxalic crystals dissolve. Slowly raise the sail while applying the acid mix. Drop to deck and let soak about 15 minutes. Slowly raise and retreat any areas not already bleached. This step will remove almost ALL of any iron stains, tannin (brown) staining, most of 'shadows' left from the mold/mildew removal in part 1.Thoroughly rinse, rinse again and again.Let sail fully dry before folding/furling, etc. Most of the dirt, mildew, artillery fungus (those thousands of teeny black spots), air pollution, iron stains, brown tannin stains will be removed and the sail should look 95% new. Caution: Oxalic acid will quickly absorb through yoiur skin and harm your kidneys .... so wear rubber gloves, foul weather gear, goggles, etc. The best source of Sodium Silicate based detergent is a Marine Market detergent - brand name: Tuff'eNuff. Made by a marine chemical Co. in Florida. Distribution is in most 'good' boat local supply chandleries on the East Coast. WM used to carry it. Stuff is expensive. NOT for laminated sails, NOT for colored dacron ....WHITE woven DACRON ONLY.