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How to Clean your Sails and Remove Surface Mold and Mildew?


Susan E. Smith asked:


My Main sail has developed mold and mildew and I’m guessing my cover is shot.
What product is safe to clean and kill mold/mildew?




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You’ll need: 1 or 2 bottles of Boat Clean Plus (1 bottle for small to medium sails, 2 bottles for medium to large sails). A medium bristled brush. A paint brush. A fresh water hose. A large container to hold your sail submerged in water, i.e. a bathtub, a 55 gallon plastic drum, a large waterproof plastic bag.

  1. Spread the sail out on a clean flat surface and spray with a jet stream of water to remove any loose dirt.
  2. While still wet, spray concentrated Boat Clean Plus on all mold and ground in dirt and soil and scrub those areas with the medium bristled brush to loosen the dirt, but not so aggressively as to damage the sail.
  3. Spray the rest of the sail with Boat Clean Plus.
  4. Flip the sail over and repeat.
  5. Flake the Sail loosely, put it into the container, fill the container ½ full with warm water and leave for 12 hours. Move the sail around and push the air out of the sail so that water is in contact with all surfaces.
  6. Spread the sail out on a flat surface or attach it to the mast. Starting at the head and working down to the tack and foot Rinse with a jet stream of water while agitating the surface with the paint brush to remove all residue. Do this on both sides.
  7. Hoist the sail and allow it to dry completely.
  8. Fabric sails can be re-waterproofed and protected from mold penetration with Canvas Shield.



This method of sail cleaning is recommended for annual sail cleaning as well cleaning mold.


Mold grows on dirt or decaying matter and can be black, brown, green, pink or yellow in color. Mildew is white in color and only grows on living matter, such as plants. Mold requires soil, moisture, warmth and darkness to grow and flourish. The best way to prevent mold is to keep the surface clean, dry and ventilated. Mold does not grow on synthetic fabrics, but on dirt that may be on the surface. Once growing on the surface, mold can penetrate microscopic cracks, pores and fissures and spread under the surface, where it becomes indelible. Keeping sails and sail covers clean and dry is the best way to prevent mold.


Boat Clean Plus is a safe, deep penetrating, free rinsing, and environment friendly cleaner and is a proven sail cleaner. Highly alkali cleaners and oxidizing agents, such as bleach can damage synthetic fabrics such as polyester or Dacron® and others that most sails are made from. Soap and detergents do not penetrate deeply enough and leave a residue which will hold dirt and is food for mold.


If sail cleaning or rinsing is done on the boat, the Boat Clean Plus residue can be rinsed from the fiberglass or painted boat surfaces and will not damage any waxed or polished surfaces on the deck or hull and is biodegradable and will not harm the waterway or soil surrounding the boat.


Wash all accessible areas of mast, boom, battens, etc. that come into contact with the sail with Boat Clean Plus and water to remove dirt and residual mold to prevent reoccurrence of mold. Sail covers need to be washed on a regular basis with Fabri-Klean and water and kept dry. Waterproof the sail cover with Canvas Shield. You can also re-waterproofPolyester or Dacron sails by spraying with Canvas Shield.


If all of the mold stains do not come out by washing and soaking with Boat Clean Plus, letting the sun get at the sail for 5 or 6 days can lighten the stains. Attempting to bleach out the stains will make the threads brittle and compromise the sail so you may have to live with some of the stains. Better a stained sail than a torn or blown sail.


If your sail cover is worn out or damaged then it may be shot. If it is dirty, covered in mold and leaking water, you may be able to restore it. Most sail covers are made from acrylic canvas, such as Sunbrella®. You may be able to clean, remove the mold and re-waterproof the sail cover using Fabri-Klean,  bleach and Canvas Shield as follows:


How to clean, remove mold and re-waterproof Sunbrella® sail covers?



You will need 1 bottle Fabri-Klean, 1 cup chlorine bleach and one bottle of Canvas Shield plus a soft bristled paint brush, 4’ to 5’ wide. Select a time when the weather will be warm and dry for at least 2 days.

  1. Lay the sail cover out on a flat clean surface, wet the Sunbrella with fresh water, spray Fabri-Klean onto the surface, and scrub with a scrub brush then rinse clean.
  2. Turn over and repeat on the other side. This will remove surface mold and dirt.
  3. If mold stains remain, mix a solution of 1 cup of 5% chlorine bleach and 1 cup of Fabri-Klean into 1 gallon of warm water and stir.
  4. Use the paint brush to apply the mixture to the sail cover until completely wet and agitate to get it into the threads. Do this on both sides. Allow to stand 15 to 20 minute and agitate with a paint brush again. Rinse well and inspect for remaining mold spots.
  5. If there are mold spots, repeat step 4.
  6. When the cover is clean allow to air dry, until completely dry.
  7. Spray Canvas Shield onto the exterior of the cover and allow to dry; 30 minutes to 2 hours, depending on temperature and humidity.
  8. When dry to the touch, spray a 2nd coat of Canvas Shield onto the exterior and allow to air dry in the sun for at least 48 hours.Canvas Shield will not affect the breathability of the Sunbrella fabric.


Your sail cover will now be clean, mold free and waterproofed for at least 1 year. You can wait until the cover loses its waterproofing and deal with it then, but we recommend annual cleaning and re-waterproofing as preventative maintenance.  It’s a lot cheaper protecting your sails than replacing them. A clean, waterproof, mold free sail cover will protect your sail from mold and stains as well and extend its useful life and of course look great on your boat.

Thanks for your question,

Captain Aurora
Richard Kittar

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