Getting Rid of Mold and Mildew Onboard

Practical Sailor tests 14 mildew stain removers, offers tips for preventing onboard mildew, and gives some advice on the proper care and cleaning of sails-Dacron, Kevlar, and nylon-to keep them mildew-free.


Among the marine maintenance products Practical Sailor evaluated recently were 14 pump-spray mildew cleaners to find out which one was the most effective at removing severe mildew stains. We tested chlorine bleach cleaners, chlorine-free cleaners, hydrogen peroxide cleaners, and ammonium chloride cleaners on a variety of materials, ranging from mildewed shower curtains to moldy vinyl seat cushions and moldy life jackets. We also used them to clean a mildewed sail and mildewed Sunbrella. All products were effective at removing the mold mildew from the shower curtain, but the cushions, life jacket, Dacron sail, and Sunbrella were more of a challenge. One product stood out as a more effective mildew cleaner: Klean-Strip Mildew Stain Remover. Klean-Strip is a highly concentrated product with 19 times more sodium hypochlorite than common bleach, and we do not recommend it for cleaning sails or fabrics. Other products tested include 3M mildew stain remover, Boatlife mildew remover, MaryKate mildew stain remover, MDR Amazons Amazing Mildew Stain Away, MDR Moldaway, Naturally Clean Mildew, Nautical Ease Mildew Stain Remover, household Spray Nine, Star brite Mildew Stain Remover, Sudbury Mildew Cleaner and Stain Remover, Thetford Mildew Stain Remover, and West Marine Mold and Mildew Cleaner.

removing mildew from sailboat

Every boatowner struggles with mildew. Not only is it ugly, but it stinks. Mildew, which is actually mold growing on fabric, plagues sails, lockers, cushions, cabin liners, life jackets, foul-weather gear, biminis-pretty much anything it can latch on to.

All mildew needs to grow is a moist environment with little ventilation, minimal light, and a food source-just about any organic matter. Before it grows, mold spores germinate on the food source. And boats offer an ample food supply: wood, paper, carpet, Sunbrella, vinyl, Dacron. When these materials become moist, its party time for the mold spores.

How fast mold grows depends on conditions like ventilation and humidity. Spores can germinate after only 12 hours in some conditions, and some grow in 24 to 48 hours. The secret is to prevent it or catch it as early as possible.

There are numerous products that claim to prevent or remove mildew. Sodium hypochlorite, or bleach, can kill fungus and mold spores if used correctly. Less-toxic chemicals like ammonium chloride are less effective at eliminating resistant molds but are less harsh on fabric and humans.

Once mildew appears, Practical Sailor recommends first trying a mix of mild soap and water to remove the mildew. The second line of defense is a chlorine-free mildew cleaner. Because some materials-like vinyl and stitching-can be damaged by repeated exposure to chlorine/bleach, its always a good idea to try removing the stains with less aggressive cleaners first (no chlorine or low chlorine). If that doesnt work-and the material being cleaned isnt affected by chlorine-try a more potent chlorine-based cleaner or a solution of 10 percent household bleach and 90 percent water to spot clean the stain; rinse thoroughly with fresh water and dry the area.

Some chemicals that eliminate mildew are considered pesticides, and may contain chemicals that can be harmful to people, animals, or the environment. For this reason, the U.S. Environmental Protection Agencys Office of Pesticide Programs regulates pesticides in the United States to protect public health and the environment. Typical pesticides found in mold and mildew removers are chlorine and alkyl ammonium chlorides. These are known as fungicides and can be very caustic, ruining some fabrics or stitching, and harmful humans.

Best Choice Klean-Strip Mildew Stain Remover

Reading the safety precautions on a product label before using a cleaner is a must. Many will recommend wearing gloves and eye protection.

The Mildew Removers We Tested

Practical Sailor tested 14 pump-spray mildew stain removers to find out which was the most effective at cleaning severe mildew stains. We do not recommend most of these be used for everyday cleaning, but the top performers are effective mildew stain removers.

Of the 14 cleaners tested, a half-dozen of them use sodium hypochlorite (bleach) as an active ingredient: Marykate Mildew Stain Remover, 3M Marine Mildew Stain Remover, Star brite Mildew Stain Remover, Klean-Strip Mildew Stain Remover, and Sudbury Mildew Cleaner & Stain Remover. The amount of bleach used varies from product to product. For instance, Klean-Strip is 96 percent sodium hypochlorite, and Boat Armor (no longer available) has 4.9 percent of the active ingredient, compared to the 5 percent found in household bleach.

Bleach-based products should never be used on nylon fabric or stitching. They will weaken the material. (See “Mildew Offense and Defense,” right.)

Budget Buy Spray Nine

Eight test products-West Marine Mold & Mildew Cleaner, Marine Development and Research (MDR) Mold Away, MDR Amazons Amazing Mildew Stain Away, Nautical Ease Mildew Stain Remover, and BoatLIFE Mildew Remover, Spray Nine household cleaner, Thetford Mildew Stain Remover, and Naturally It’s Clean Mildew-are chlorine-free. Naturally It’s Clean uses “pure vegetable-based enzymes” and coconut oil as its active ingredients, while Thetford contains hydrogen peroxide, and Spray Nine uses ammonium chloride.

While several test products claim to be environmentally friendly (see comments on Value Guide, top right), only the Thetford cleaner has met EPA standards to carry the agencys Design for the Environment (DfE) logo.

The Best Mildew Cleaners

We tested the cleaners on a variety of materials, ranging from shower curtains to vinyl seat cushions, life jackets, and sails (see “How We Tested,” above). All were effective at removing the mildew from the shower curtain. The life jacket and seat cushions proved to be more worthy opponents for our test group.

As we often do when testing large groups of boat maintenance products, we narrowed the field after the first round of testing, and let the top products duke it out in a clean-off.

In this test, the group of finalists was a large one: Eight cleaners did well at cleaning the vinyl cushions and moved on to face the life jacket, which was covered with dark black mildew stains. They were Spray Nine, Klean-Strip, Marykate, MDR Amazons Amazing cleaner, 3M, Star brite, and Sudbury.

Only one cleaner was effective at cleaning the life jacket: the high-chlorine-content Klean-Strip Mildew Stain Remover, a product manufactured by W.M. Barr & Co. in Memphis, Tenn. The website describes the product as a “professional strength cleaner.” The Klean-Strip line includes paint strippers, thinners and solvents, rust remover, and gum and adhesive removers.

best mildew removers for boats

None of the products caused any apparent damage to the life-jacket material.

Practical Sailorprefers products without harsh chemicals-even if they require a bit more elbow grease. But none of those tested were so caustic that they were hard to work with, although the chlorine-based products did have a discernible bleachy smell.


Your best defense against mildew is prevention, but once those black stains appear, act fast. To avoid the possible damage that repeated exposure to harsh cleaners can cause, try to clean the stains first with soap and water, then a non-chlorine product.

Testers favorite bleach-free cleaner in this test was the Spray Nine. A top performer with the best price, Spray Nine also notched Budget Buy honors. Another top chlorine-free cleaner was Nautical Ease.

If the mildew problem is too tough for mild measures, advance to a diluted bleach-and-cold water solution-usually the cheapest approach-or a chlorine-based cleaner.

The hands-down top performer in this test was the bleach-based Klean-Strip. It stood out from the rest of the pack, easily cleaning the vinyl cushion and making a significant impact on the life jacket. Its the Practical Sailor Best Choice for cleaning extreme mildew stains, but with caveats. This is a highly concentrated product-it has 19 times more sodium hypochlorite than common bleach-so be sure to read its label carefully and be selective about what surfaces and materials you use it on. We do not recommend using it undiluted for cleaning sails or fabrics.

Among the other bleach products that performed well, Marykate and Star brite are reasonably priced.

For the greenies out there, your best choice is the Thetford. While others claim to be eco-friendly, only Thetford has met the established DfE standard.


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