Here's a process for woven dacron sails that Ive been using for many many years for outstanding results. Its similar to the same 'washing' process for new (greige) polyester fabrics during manufacture.
This process extracts most dirt, funguses (those millions of 'teeny black spots') and 'brown' staining from tannins and iron.
For WHITE Woven dacron only, NOT for dacron 'laminates' or other laminates:
Note - Its best to clean the sail ON THE BOAT; but, the boat cannot have any exposed bare exterior teak or other bare wood including large areas of imperfectly varnished, etc. wood, bare teak decks, etc. as the 'detergent' will extract tannins from the bare wood ... including 'pin holes' and dings in the surface coating (varnish/Cetol/etc.). All 'canvas', etc. should also be removed.
This process is NOT used for 'colored' nor dyed sail material.
Due to the potency/strength and high 'alkalinity' / caustic nature of sodium silicate based detergents you really should/must be wearing eye protection, gloves, and foul weather gear - Sodium silicate detergents will begin to dissolve your skin/corneas, etc. so BE CAREFUL and wear 'protection' - goggles, gloves, foul weather gear, etc.
When bleaching in step 2 - you MUST have this protection on as oxalic acid will quickly absorb through your skin and form sharp crystals in the nephrons of your kidneys - instant kidney stones / kidney damage is a possibility without 'protection'.
Dacron polyester is immune to attack by sodium silicate and other strong 'caustic' detergents.
Detergent: a strong sodium silicate based detergent - ie.: 'Roll-off', 'Tuff-eNuff', etc.
Bleach: Oxalic Acid Crystals (wood bleach - from a paint or hardware store)
On a WINDLESS, high humidity, and cloudy day ....
Preparation
• CLEAN THE DECK thoroughly, as woven dacron is a 'dirt magnet' especially when wet.
Step 1
• Slowly raise the sail while spritzing on the detergent and while spreading the detergent with a 4ft-6ft. long handled soft bristle scrub brush. Apply to both sides as you raise the sail. Spritz with water as needed to keep the sail fabric 'wetted'
• Immediately drop the sail to the deck and cover with a plastic tarp to prevent the sail from drying. Let the sail soak for approx 20± minutes - soak time is important, especially for the extraction and dissolving of embedded fungals (all those teeny 'black spots'), oils, and most 'dirt'.
• Slowly raise the sail as you scrub with the long handled soft bristle brush. Apply additional detergent as needed to keep the sail 'wetted'. Spritz with a garden/dock hose as needed.
• Drop the sail back to the deck and cover with a plastic tarp, and let soak for another ~20± minutes
• Repeat raising, scrubbing, wetting, soaking for another 20± minutes.
• With dock/garden hose (open at 'full blast') slowly raise the sail while 'blasting' with the water hose.
• Repeat cleaning with detergent, etc. on the small areas of 'stubborn' dirt, etc. .... stains will be removed in step #2.
• Raise and repeatedly RINSE until ALL 'suds', detergent and liquified dirt etc. is removed, then recover with tarp.
Its important to NOT let the sail 'dry out' during any portion of the above.
ALL ' caustic detergent MUST be removed and the sail THOROUGHLY rinsed before proceeding to step #2 !!!!
Step 2 - Tannins, iron staining extraction and bleaching:
Note (REPEAT) - ALL 'detergent' MUST be fully rinsed out of fabric before proceeding with below steps.
• Obtain a 'pint' of oxalic acid crystals (wood bleach) from paint or hardware store.
• pour 'most' of the crystals into an empty (2 gallon / 4-5 litre) bucket and then slowly pour HOT water into bucket. The goal is to not dissolve ALL the oxalic acid crystals - you should use just 'enough' HOT water so that there are a few undissolved crystals in the bottom of the bucket (a 'saturated' oxalic solution - in 'chemist speak')
MUST have full protection ON for the following steps !!!!!!
• Slowly raise the sail while applying oxalic acid solution with long handled scrub brush to both sides of sail ... bleaching should be 'almost instantaneous' (if you extracted 'most' of the dirt in step #1); drop sail back to deck and recover with tarp or repeat raising and applying if staining remains.
• Raise, lower while rinsing several times with water to extract remaining oxalic.
• Raise fully rinsed sail and let fully dry ... or go sailing until dry.
Other
• Apply unused oxalic acid solution to water-line/topsides/bow to remove 'brown mustache' caused by tannins and iron in the water.
• Re-wax the boat !!!! - sodium silicate based detergents will extract and 'strip' wax from gelcoat, etc. ... such old 'dead' and oxidized wax should be extracted periodically anyway, as such promotes and accelerates surface oxidization of the gel - IMO.
hope this helps
