PS Advisor June 2013 Issue

Anti-Mildew Weapons

Can bottom paints fight interior mold and mildew?

I was wondering whether you might have any comments on using bottom paint on the boat’s interior to help fight mold and mildew. Many bottom paints are ineffective out of the water, but I was wondering whether some of the new eco-friendly paints might prove to be a new weapon in the fight to work less and play more. 

Barry Simmons
Lighthouse Point, Fla.

We talked with a few major marine antifouling manufacturers, and they agreed that applying bottom paint to a cabin interior is not a recommended practice. Bottom paints are not formulated for interior use or to prevent mildew. Most come with labels warning against application in small spaces. Even the VOC-free, biocide-free, water-based “eco-friendly” paints have some risk when used in small confines. Aesthetics is another reason to leave bottom paint for the exterior: Antifouling paints lack gloss and would be a bear to keep clean; they absorb stains and odors more readily.

Photos courtesy of manufacturers and by Ralph Naranjo.

There are a number of better options for fighting mildew onboard—and they have fewer health risks and are less expensive than bottom paint.

Every anti-mildew strategy should start with keeping the boat dry and well-ventilated. Good air circulation and moisture control go a long way toward preventing mildew. To maximize ventilation, look into adding dorade boxes (PS, May 1997), solar hatch vents (PS, May 1, 1993), or 12-volt fans (PS, April 2008).

We recommend Marinco’s Nicro Day and Night solar-powered vents. During the day, a solar panel charges a small Nicad battery that powers the fan at night, giving you continuous circulation. The top cabin fans in our last test included the Hella Turbo (, Caframo Kona (, Caframo Sirocco, Hotwire Port Fan (, and the Fan-tastic Vent Endless Breeze ( Also check out the new Caframo Taku, reviewed in this issue.

For moisture control, consider using absorbent desiccants (PS, November 2012) or a dehumidifier (see page 14 of this issue). Which chemical drying agent is best for you will depend on your boat and the planned application. We recommend the calcium-chloride Damp Rid ( for closed-up boats, and Nordic Power’s Nordic Dry ( or Pindell Engineering’s H2Out silica-gel SD309 ( for spaces like large lockers. On problem boats in cooler climates, we also recommend fogging the boat interior with Concrobium prior to winter storage.

If you’re planning to re-paint your cabin anyway, Practical Sailor Technical Editor Ralph Naranjo (a former boatyard manager) recommended using a latex house paint that’s formulated to keep mildew at bay to paint lockers and other hidden interior hull surfaces. PS Editor Darrell Nicholson had good luck using Rustoleum’s Zinsser Perma-White ($10/quart) to protect the cabin of his wooden ketch. Rustoleum ( claims the mold- and mildew-proof paint is moisture resistant and blisterproof. The company guarantees the water-based, self-priming paint will be mildew proof for five years. You can find it at Lowe’s,, and Sears. 

There are also mildewcide paint additives that can be mixed with latex or oil-based paints and applied to a cabin interior. Rustoleum makes one called Add-2 Prevent Mildew Mildewcide, and Krud Kutter ( markets a liquid, mildew-fighting additive called Supreme Chemical MC-2 Di-All; both are available online.

To protect your onboard gear—like life jackets, foul-weather gear, and cushions—you can use a spray-on preventative like those we tested in the June 2010 issue: Concrobium, Mold Off, and 3M Mildew Block. All did well in testing, but Concrobium was the leader of the pack. It’s a safe, economical spray protectant.

Comments (3)

Rustoleum also makes a commerically available USDA aproved mildew proof paint for dairies. I got mine from MSC ( and it has worked for seven years. Quinton Roesser, Mariah

Posted by: quinton r | July 25, 2013 5:47 AM    Report this comment

You must keep these vents going at all times. You should have for example two rear cowl vents open (screened for bugs) and then two roof vents on the cabin roof and one on the forward hatch. Pull the air from the rear intake cowl vents. Also install a mast boot at the foot of the mast to keep out rain water.

Posted by: Michael B | May 22, 2013 9:26 PM    Report this comment

I use two Nicro Day/night vents with the front vent installed in my forward hatch being the intake, and my mid ship's hatch being an exhaust. They don't move a lot of air, (atleast it's not that noticeable) but I feel like it will help keep it from getting stuffy inside if we ever leave the boat closed up for an extended period.

Posted by: Dustin K | May 18, 2013 4:34 PM    Report this comment

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