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Simple and effective process for cleaning "DACRON" sails; NOT for mylar or other film laminated sails or colored dacron:
Obtain a strong detergent containing "Sodium Silicate" - typically from a janitorial supply or at chandleries a product called "Tuff-eNuff". Go to a hardware store and get some Oxalic Acid crystals (Wood Bleach). Get a 'soft' long bristle brush with a 6-7 ft. handle.
Do this on a windless/cloudy day with high humidity so that the detergent doesnt 'dry-out' while you are working ..............-
Dont do this on a boat with a bare teak deck, etc. as this process will strip the tannins out of the teak !!!!!!!!!!! This process is for WHITE *woven* dacron fabric .... dont do this if colored fabric, laminated sails, etc.
Wear goggles, rubber gloves and foul weather gear to protect yourself from the 'strong' detergent.
1. Clean the deck of the boat.
2. Drop the sail to the deck, then raise about 4 ft. and spray on the detergent and spread the detergent with the long handled brush so that the fabric is totally wetted, spritz with a mist from a garden hose - not to soak but to just keep the sail 'wetted'. Do the 'reverse side' in the same manner.
3. Continue to raise the sail in 4 ft. increments while applying detergent and mist.
4. When all of sail is wetted and has detergent applied, drop to deck while 'misting' and then cover with a plastic tarp to prevent drying out. Let sit under the cover for 1/2 - 1 hour.
5. Slowly raise the sail while scrubbing with the long handled brush, add water mist as needed to keep it wetted, drop the sail and do the 'other side'. Don let the sail dry-out.
6. cover with plastic tarp for about 1/2 hour, then raise and scrub again if needed.
Note: The sodium silicate will release most of the mold/mildew stains (actually it dissolves the cells of the mold/mildew - but may leave a 'shadow' which will be removed in subsequent steps. Most of the teeny black spots you find on woven dacron sails are colonies of 'artillery fungus' ... readily removed by a sodium silicate based detergent. The 'soak time' is important as these cels are dissolved by the Sodium Silicate.
7. Slowly raise the sail, scrub and flood the sail with a garden hose to release the mold and dirt, etc. Raise in 4ft. sections, do both sides.
7a. You may need to raise and lower several times to remove ALL the detergent ... When there is NO detergent (soap bubbles) left then proceed to step #8. ...... This is important as you dont want to mix the chemicals of the above step with what comes next.
8. Mix up a saturated solution of water and oxalic acid crystals. About 3-4 gallons of hot water into which you add oxalic crystals of sufficient quantity so that no more crystals dissolve .... add enough crystals so that no more dissolve and you can SEE the undissolved crystals on the bottom of the bucket.
Note: Oxalic acid is a potentially dangerous chemical that will absorb quickly through your skin and deposit in your kidneys ... you must wear gloves, goggles and foul weather gear.
9. dip the brush in the oxalic solution and apply to the sail as you raise it in 4ft. incrrements ... one side at a time.
9a. drop the sail and do the other side.
Note: you dont have to scrub hard when using oxalic as its a 'bleach' and will quickly beach out rust and tannin stains (the shadows of the mold/mildew, etc.).
10. Raise and lower the sail several times while flooding/rinsing with the hose to completely remove the oxalic.
11. raise the sail to thoroughly dry, or go sailing
The above process should return the sail 'almost' to new condition, will NOT harm normal woven dacron material or polyester sail thread, wont 'beat the hell out of the fabric, nor destroy the filler in the fabric.
The problem with dacron polyester is its ability to 'hold' dirt and mildew that is hard to release. The soak times above are important so that the detergent has 'time to work'.
Note: The above process will have stripped ALL the wax out of the gelcoat on the deck and where it runs over to the topsides .... which you probably should strip such wax about every two years anyway as old dead wax accelerates the 'ageing' of gelcoat .... simply rewax the gelcoat to 'seal' it to prevent oxidation.
Note: use the leftover oxalic solution on the water-line to remove the accumulated tannin stains in the gelcoat ("moustache") ... then seal the 'pores' of the gelcoat with wax to prevent future 'moustache'.
Ive been making my own sails for the past 30+ years and this process of cleaning is the best/easiest/fastest that Ive found to clean a woven dacron fabric sail. Dont do this on colored or laminated dacron ... or anywhere near bare teak.
Hope this helps.