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If the sail is mace from white woven dacron ......
Clean the sail on the boat, as woven dacron is a 'dirt magnet' if you clean it on any non-clean surface. Cleaning it on the boat will prevent dirt-pick up 'during' cleaning. (If you have bare teak or bare teak decks dont do this as the followiing method will extract the tannins from the teak and this will transfer to the sail.)
A SodiumSilicate based detergent - I use a product named "Tuff-eNuff" made by Wallace & Sons (FL) ... found in many East Coast boat chandleries. WM can 'special order' - expensive.
Oxalic Acid crystals - from a hardware or paint store ('wood bleach').
Large plastic tarp.
SOFT scrubbing brush on long pole.
Step #1 .
On a windless cloudy, high humidity day, slowly raise the sail as you spray on the detergent, spreading it as you go. Quickly continue until the sail has an even coating of detergent until the sail is fully raised, then immediately drop it onto the plastic tarp, cover the remainder with the tarp - this will prevent the detergent from drying .... let soak covered for an hour.
After an hour or so, slowly raise while scrubbing with the soft brush and respraying any area that has 'dried', until the sail is 'full-up', then drop back and cover with the tarp. Repeat raising while scrubbing concentrating on the areas that are especially cruddy and let soak under the tarp.
With dock hose in hand slow raise as you rinse ALL the detergent out of the fabric. Repeat until NO evidence of detergent is left in the fabric. The detergent MUST be totally removed before the next 'treatment'.
This first step will release 99% --- dirt, dissolve any 'artillery fungus' growing (those pesky teeny black spots), 'air pollution', etc. but there will be 'yellowish' or iron staining remaining - extracted in the next step. Expect the rinse water to be 'quite filthy', keep rinsing until the rinse water is 'clear'.
Step 2 -
Mix the oxalic crystal into WARM water. Start with a few quarts of warm water and add the oxalic to the water so that most, but not all, oxalic dissolves in the water (saturated solution) keep adding oxalic so there are a few undissolved crystals in the bottom of the bucket.
Slowly raise the sail as you apply the oxalic mixture, this will bleach out the tannin, mud, shadows of previously removed artillery fungus, rust, etc. staining not extracted by step #1. The 'bleaching' will be almost instantaneous. Then rinse SEVERAL times to make sure that 100% of the oxalic is removed.
Then leave the sail 'up' until fully dry .... or go sailing until dry.
Caution - Sodium Silicate detergents will slowly dissolve skin, eyeballs and other 'organics'; Oxalic will rapidly absorb through your skin and cause harm to your kidneys
... so wear rain gear, gloves, goggles, etc.
This process will also remove the wax from your gelcoat, etc., especially the old dead wax ... so you will need to rewax the boat (in all probability the old dead wax 'should' be stripped out yearly anyway to retard oxidation of the gel). BE sure to rinse thoroughly, especially between steps #1 & #2 as sodium silicate detergent and oxalic acid are not 'compatible'.
The result will be a sail that looks almost brand new. These 'chemicals' will not harm, etc. polyester dacron nor the dacron polyester stitching, etc.
** Do NOT use these on dacron 'laminates', dacron cruising laminates, nor any other 'laminate' sail, nor any sail that is other than WHITE.