Mold & Mildew

By Carolyn Shearlock © 2016 • all rights reserved

Mold and mildew are the bane of every boat owner in hot, humid climates. Tips to minimize the problem when "the old stand-bys" just don't do it.

Uggh. Mold and mildew. How do you get rid of them in hot and humid climates?

I don’t have the perfect answer. What I do know is that boat interiors seem to be fertile breeding ground unless you’re running an air conditioner or dehumidifier. NOTE: I am NOT talking about any types of toxic mold here . . . just “normal, everyday mold” that shows up in boats in humid climates.

When we were cruising in the Sea of Cortez, we didn’t have a major problem with mold and mildew as it’s relatively dry there (the Baja peninsula is a desert). But when we spent a summer in El Salvador, we had a bit of a problem. This winter, in the Florida Keys, was the worst I’ve experienced . . . and from the email I’ve gotten, it seems that many people were fighting mildew all over the US and through the Caribbean.

Here’s what I’ve learned, first in the area of prevention and then in cleaning:


We haven’t found anything to be 100% effective in prevention of mold and mildew. Three items that are frequently mentioned – bleach, vinegar and tea tree oil – did absolutely nothing for us (ditto for many other cruisers I’ve talked to in south Florida). What has helped:

  • Really improving ventilation with fans, exhaust fans, wind scoops and just keeping hatches and port holes open.
  • Moving things around and getting rid of even more clothing, bags, extra towels and bedding so that there is more unrestricted air movement in the boat as well as less “stuff” to get mildewy.
  • When possible, washing anything that did have mold or mildew on it in very hot water, which does kill the spores.
  • Do laundry more often, particularly of sheets, towels and anything else that may get damp or sweaty. Again, I use hot water whenever I can.
  • Hang towels and rags out to dry whenever the weather allows (don’t let them sit damp in the boat).
  • Rinse anything that’s been in salt water off before coming in the boat (salt attracts and holds moisture): feet, shoes, swim suits, dog. Unfortunately, sometimes we have a wet dinghy ride and our clothes get soaked . . . and it’s just not feasible with other boats nearby to strip off all the wet clothes on the back deck! But we try to stay as close to the door as possible and toss the wet clothes into the cockpit so the salt water doesn’t get in the boat.
  • We use Concrobium liberally on fabric, wood and fiberglass. It’s sold in home improvement and hardware stores in the PAINT department, not cleaning supplies. Spray it on and as it dries it kills the mold and mildew spores, and continues as a preventive. You’ll see reviews where people say it totally stopped mildew but that’s when they were able to eliminate the conditions that led to mildew . . . on a boat, it seems to help a lot, but since you can’t eliminate the humidity, mold and mildew will eventually come back if you don’t keep re-treating. About once a month, we go through the various areas of the boat and spray, moving storage crates and cleaning them as we go, too (note: you have to leave it wet so it kills the spores as it dries). If you can’t find Concrobium in a store near you, you can get it on Amazon:

Mold and mildew are the bane of every boat owner in hot, humid climates. Tips to minimize the problem when "the old stand-bys" just don't do it.

  • Concrobium is safe to use on fabrics and we find it has helped a lot with mildew on our life jackets, foul weather gear and hats. No, it hasn’t totally eliminated problems – but they are far less severe than before we were using it. We also are making a big effort to let all these dry outside before we bring them inside the boat.
  • In areas where air just doesn’t move (inside a couple of lockers in the bow and stern of the boat), I’ve also  hung a “Sun Fresh Crystal Disc Mold Inhibitor.” It seems to be helping, along with everything else we’re doing, but I have no way of knowing for sure. Buy them on Amazon. One will last about 9 months and slowly disappear into the air. Replace when there is nothing left but the paper.

Mold and mildew are the bane of every boat owner in hot, humid climates. Tips to minimize the problem when "the old stand-bys" just don't do it.


Mold and mildew stains are tough to get out of fabric and wood. Items that can go in a washing machine with hot water do pretty well although there may still be a little stain left. Items washed in cold water don’t do nearly as well.

Concrobium isn’t marketed for removing stains, but we’ve found that it helps considerably with upholstery and “fake leather.” It does a great job on fiberglass and plastic surfaces – spray it on, wipe off, then respray and let dry to inhibit further growth.

Bleach can help some items but can cause its own problems with discoloration. If you want to try a dilute bleach solution (I use about 10% bleach – or less), test it first on a hidden spot.

There are numerous “mold stain remover” sprays on the market. Most work with bleach. We haven’t had much more success with them than with just using bleach or concrobium but depending on the material you’re working with, you might. Most of these are sold with cleaning supplies in home improvement stores and hardwares.

If you are particularly sensitive to mold, have a lot of it on your boat or have respiratory problems, I’d suggest using a sanding dust mask at a minimum . . . or perhaps a true respirator if you have one.


If you’re in a hot and humid area, you’re almost certain to have to deal with mold and mildew. And it won’t be a one-time battle: it will be ongoing. You can kill off the existing mold and mildew, but as long as the conditions exist, it will keep coming back. The tips here will lessen the problem considerably, but I don’t know of anything that will eliminate it permanently while still allowing you to be aboard short of air conditioning or a dehumidifier . . . and those aren’t practical without shore power.

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  1. How do you keep a mattress from getting wet on the bottom during the winter months? Have you ever had this problem?

  2. Ann Robinson says:

    I swear by ‘oil of cloves’ – it is very powerful and you only need about 10 drops in a litre of water. It DOES kill mould spores so can be used to spray surfaces including upholstery when you are leaving a boat in storage and it will also remove mould – you can add a few drops of detergent to the solution for cleaning

    • Absolutely right Ann, it seems to be the only thing that works for us in tropical Australia. And your recipe is our recipe.

      • I forgot to add, we also use this at home. The timber walls in the house used to go black with mould in the wet season. We now spray the walls with the mixture at the beginning of the wet season i.e early November, let them soak it up for 5 – 10 minutes, then wipe with a damp cloth. Good bye mould.

    • As we don’t like to use too many chemicals in our house and on our boat we solely use clove oil, even to ‘clean’ (aka ‘de-mould’) our leather upholsteries after the wet season – works like a treat, is absolutely non-toxic, smells nice and has been used by generations of Australians in hot, tropical conditions. Usage instructions are above, and see you later, mould.

  3. Charles Martel says:

    Best way to get Dri-Dek is direct from them at It’s better than the other stuff because of the anti-mold stuff in the Dri-Dek material. I’ve used it for years.

  4. Pam Cogan says:

    I use a high pressure steam cleaner in the spring, removes mould, bacteria and smells very well. Can be rented from Rentall, or other equipment rental center. Because its steam uner high pressure, it dries very quickly, and does not leave everything moist like you would expect. But I always leave the cabin wide open and do it on a sunny dry day anyway. Using steam is effective to kill basically every micro-organsim on contact (no “wet” time required like every other cleaner) AND its completely safe for the envirnment, especially the sensitive marine environment. Strong mildew/mould cleaners are extremely damaging to the waters we love to sail/cruise. Even if you’re careful, some of it will end up in the water. Products that claim “environmentally friendly” are probably not safe for marine environments, even plant products. Great site, though, thanks for keeping it up!

  5. We have a mold problem in our shower drain and one thing that helps is boiling water. I keep an electric tea kettle nearby and when running the shower to get the water warm, I catch it in the tea kettle. The next time we run our generator, I boil the water in the kettle then pour it down the drain.

    It isn’t a permanent solution, but it helps and is completely nontoxic and environmentally friendly.

  6. Any suggestion for boat storage in Pacific NW? Keep it in covered storage area, but generally keep everything closed up. No problem with mold as of yet, but then again don’t want to have one at all!

    • We use a product called Homezone which we order online at to prevent/kill mold while the boat is stored in Florida for the summer. It lasts for 6 months, we use the whole container placing paper plates all over the boat including the lazzerettes and engine room. When we return in the fall most of the odor is gone but we leave everything open for a couple of hours to be sure. The boat is as clean as we left it in the spring. This was recommended to us by another cruiser who has used it for several years…this will be our 3rd season. This is the last thing we do before locking the boat. Within a few hours the fumes are pretty toxic…its a form of formaldehyde. Use mask and gloves when handling.

      • I’ve heard good things about it, but if you use it while the boat is stored, you can’t have a boat watcher check the inside of the boat because of the fumes AND should not be used by anyone with breathing/lung problems.

  7. Lorraine says:

    Re bleach stains which on some surfaces look a dark cream colour. Apparently the stain can be removed with silver jewellery cleaner. Haven’t tried it myself but believe it works. Would pay to patch test first.

  8. kriskret says:

    Kalium Hypermanganicum (Potassium Permanganate) water solution. Just make pinkish colour solution – few tiny crystals with water , and spray. – Kills viruses , bacteria , spores, mildew on contact. Buy it at your local pharmacy. Store it in glass jar , prevent from moisture !

  9. I have successfully used a strong concentration of oxyclean and hot water soaked for a day or two to remove mold & mildew from fabrics. Hope this helps ?

  10. Concrobium also makes a product called Mold Stain Eraser which is a powder that you mix and spray. I haven’t tried it yet on boat walls where spots remained after cleaning, but will report back after I give it a try. Now that we use a powerful dehumidifier, our moisture problems are gone (in our old wooden boat)

    • Judy Cook says:

      I just found this product and used it on very dark black mold stains on the ‘monkey fur’ fabric under our windows. After a good dose, the stain practically disappeared!!! The powder was not particularly easy to use… even in hot water the crystals didn’t dissolve and clogged my sprayer. I soaked a sponge and used it to apply to the stains wetting thoroughly. The darkest stains required more than one application.

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