If these are WOVEN dacron sails they are quite easy to clean ... while ON the boat at a slip, etc.
The following doesnt apply to colored/dyed dacron, nor dacron taffeta laminated, nor other laminated sails ... just old fashioned woven white dacron. Note: The boat cannot have any exposed non-coated TEAK such as teak decks or 'grayed teak' ... as the above 'chemicals' will strip the tannins out of the bare teak and make a real mess.
You will need- soft bristle scrub brush with 6+ft long 'handle', garden/dock water hose with adjustable 'nozzle', plastic tarp, Sodium Silicate based detergent in a spray bottle ( I like the brand - Tuff-eNuff by Wallace & Sons), oxalic acid crystals from a paint/hardware store, rubber gloves & goggles, etc. for self protection. Dont use 'oxalic acid' substitutes ... found in AGGRESSIVE eco'-states.
On a cloudy, humid day with little to NO wind ....
Raise the sails a few feet at a time and 'spritz' a 2-3ft high section at a time with the Sodium Silicate detergent ... plus a wee bit of water spray from the hose; spread the detergent with the soft scrub brush. Do this to both sides of the sail. Continue in 2-3ft. 'raises' of the sail until the sail is fully saturated with detergent and 'mist' water.
Quickly drop the sail to the deck and cover with the plastic tarp to prevent the sail from drying out - wait one hour. It takes TIME for detergents to loosen and emulsify dirt, etc.
Scrub the sail as you raise and reapply detergent and 'mist' to those place not previously fully soaked. When all the sail surface scrubbed, 'mist' the sail with water and drop back down and recover with the plastic tarp - wait another hour.
Raise and rescrub until all the sail is scrubbed, drop and re-cover and wait another hour.
Raise the sail as you BLAST with water (stand back as the detergent will dissolve skin, eyeballs, etc.) as you raise the sail to full up; Repeat the raising and rinsing until there is no HINT of detergent/foam, etc. and the rinse water is 'clean'. This step will release a most of the organic dirt, etc. from the sail.
Bleaching step to remove iron stains, tannin stains, mildew 'shadows', etc. etc.:
Put dry oxalic crystals (about a 1/2 pint, dry) into a bucket and mix with HOT water. Add 'just enough' hot water so that there are a few crystals undissolved in the bottom of the bucket. Rinse the brush thoroughly with water to remove all previous detergent and Apply the oxalic mixture to the sail in the same manner as the detergent. Most tannins and rust stains will be gone in a few seconds, do the whole sail, drop and let soak, as before. Then, thoroughly raise/rinse a few times to ensure that all the oxalic is removed.
Caution: oxalic acid is quickly absorbed through your skin and can 'reform' in the nephrons of your kidneys as very sharp and very painful crystals. Wear protective gear when applying / handling oxalic acid mixtures.
Let the sail fully dry before lowering or go sailing to let it dry.
Cost: Tuff-eNuff ~$15.00 per quart spray bottle, oxalic crystals ~$7.00 / pint measure.
Result: will be equal or better than a 'pro' cleaning.