Saturday, December 24, 2016

Mildew Treatment for Pennies

The great myth of boat ownership--other than believing that everything takes 3 times as long and costs 4 times as much as you expect--is that mildew is ubiquitous. No matter how leak tight, no matter how well maintained it is always there. Well, I disagree wholeheartedly, and I challenge anyone to find any in my cabin. How have I dodged this scourge?

First, keep the boat leak-tight. That means no water in the bilge and no leaks around deck hardware. Not that hard if you mount things right. Strong enough so they don't move, bedded with polyurethane caulk or butyl rubber.

Second, if there is a leak that starts some growth, treat it right. In fact, I've learned far more at home, cleaning a basement that has fallen victim to occasional flooding, than around boats. The key is a cleaner with the following characteristics:
  • Controlled alkaline pH. Mold and mildew prefer slightly acid conditions. While vinegar has a faithful following, I was able to demonstrate in head-to-head testing that in damp conditions that alkaline treatments are more effective.
  • No food. Again, vinegar is a problem because it becomes mildew food when the damp returns and can actually actually accelerate growth. Likewise soaps and detergents are a problem; the mildew uses them as food.
  • Can be left in place and NOT rinsed off. Or rather the rinse must contain the inhibitor. For this reason, do NOT increase the dosage in the hope that more is better. It isn't.
  • Contains an additional agent that is toxic to mildew. In the second formula, borax is a powerful anti-mold and anti-bacterial.
  • Not bleach. While bleach can be effective on the surface, it is damaging to many surfaces, first as bleach, and then when it dries, because the pH is far too high.

You could troop down to Home Depot and pay many dollars per gallon for pennies worth of chemical in a bottle. Plastic, shipping, mark-up and and paying for know how all cost. Of you could simply brew up something proven to be more effective.

Unlike bleach, both of these formulas require some scrubbing. Some pre-soak time helps, killing the organisms and loosening the bonds. After that, a little elbow grease. If you need to rinse, remember to re-treat to provide protection from re-infection.

(First seen in Practical Sailor, October 2013

 Concrobium is a top performer in many independent tests.It is also dead simple, easily formulated from stuff you can get at stores you already go to.

DIY Concrobium knock-off formula
  • 1 quart hot water
  • 1 tablespoon baking soda (sodium bicarbonate)
  • 2 tablespoons washing soda (sodium carbonate)
  • 2 tablespoons trisodium phosphate (TSP)
This works better than all of the commercial formulas on natural fibers and 90% of the commercial formulas on synthetics (there are a few specialty formulas based in silicone quatrenary amines that are more effective on synthetics). But in head-to-head testing using canvas strips in special mildew chambers and on old PFDs I treated in strips and then left under a backyard shed, the clear winner was always a borax-based cleaner of my own formulation. Again, the key is to maintain the correct concentrations, don't add any detergent, which will only become mildew food, and then leave the final rinse of this treatment to dry in place. For most cleaning, all the is required is to spray the area until wet, scrub vigorously, and wipe off the excess.

Borax Mildew Treatment
  •  1 quart hot water
  • 2 tablespoons baking soda
  • 2 tablespoons Borax
  • 1 tablespoon TSP


6 months later

     This has been a lifesaver on wet basement carpets. Last time we had a flood I had injured my back and was unable to get the carpet cleaner out for several weeks. It had begun to reek of mildew. However, going over the carpet with this borax formula, once as a cleaner, and then a second time as a rinse, not only removed most of the mud and mildew, but also killed the smell and prevented all growth, even though the carpet stayed wet for a few more days. The stuff is a miracle.

    BTW, it is also very effective for cleaning mildewed drywall before painting. The mildew will be killed, it will not return, and the residue will not affect paint adhesion.

    Why is not sold in the stores? One reason is that claiming it kills mildew would require registering it as a pesticide. So long as common chemicals like borax are sold as generic they are exempt, but the moment you formulate and make claims, the regulatory status changes.

    But the real answer is that I don't know. I can only assume that the sellers of cleaning agents believe folks will buy a bleach based quick-clean product, but can't understand the benefits of prevention. They may be right. But I think sailors can understand.

    So this is my gift to you for the holiday season. The most effective anti-mildew cleaner avialabe for pennies. Enjoy.

    Merry Christmas!


    1. Hi Drew,
      Can the dry ingredients be mixed ahead of time and stored dry? One container of the dry ingredients rather than three.
      Thanks for the recipe and happy holidays,

    2. I don't see why not. They won't react dry.

    3. Will this work on wood decks?

    4. This is not the most effect cleaner for wood decks. I did test this, along with some other DIY formulations for a article in Practical Sailor called "Spiffy Teak for Penny Pinchers."

      The first thing to remember about teak is that all cleaners that will really restore the original appearance do so by eating off a surface layer of up to 0.01-inch. Do that very many times, and your deck gets thin. Thus, the best practice is to clean it by scrubbing across the grain with seawater.

      If you want to do a little better, try dilute ammonia. The damage is minimal and it is reasonably effective.

      Finally, commercial cleaners are based on oxalic acid. That eats off the dirt and a surface layer. The second step is a nuetralizing treatment with some cleaning ability. If you want to DIY, oxalic acid can be purchased on-line (1-cup per quart). The second step is borax (1/2-cup per quart). However, like any strong, restoring cleaner, this should be used very rarely, not even every year.

      My suggestion is to subscribe to Practical Sailor Magazine.

    5. Do you think the borax recipe is safe for sails? I have a genoa in great shape but it's gross with mildew. Would love to just clean it up and keep using it.

    6. Yes, borax is safe for sails at the recommended dose.

      However, if the mildew does not come off with light scrubbing, do what the pros do: Oxiclean powder and an overnight soak. Blend about 1 ounce per gallon of warm water and leave it at least 4 hours, agitating occasionally. That will remove about 80% of it. The sun will ten remove the last part over the next few days. There are a number of commercial sail cleaning formulations available, based on percarbonate, just like Oxiclean. This is safe for sails and approved by both the fabric makers and the sail companies.

    7. My brother used this formulation in his RV to great effect, and I've used it to wipe down as much of my boat as I can reach, undersides inside cabinets etc. Very effective, and also in the bathroom at home! Thanks very much!

    8. Drew, I recently bought an underused sailboat whose running rigging is both dirty and green with algae. What are your thoughts about using your formulations on these lines?


    9. Yes, this would help with algae. After soaking it to get it mostly clean, I would change the water, add Oxyclean (NOT bleach) and soak for another 3-4 hours. That should do it.

    10. Thanks Drew! Is Formula B safe on finished teak interiors? I guess overall. where shouldn't it be used?

    11. In general, any alkaline (which this is) cleaner can dull gloss paints and varnish if left in place to dry. I would wipe it off. If it is an oiled finish, on the other hand, it is one of the better treatments. In fact, Halberg Rassy specifies a borax-based treatment for teak decks.

      Another effective anti-mildew treatment, safe on ALL surfaces, Chloro Algae Eliminator (, sold for pools. It is NOT bleach, but is benzalkonium chloride, used as an antibacterial in many hand cleaners. It is also good on sails, for soaking ropes, and killing algae on the dodger or sail cover (allow to dry on the sun for a week--then clean off). It is not a cleaner, but it is an inexpensive and highly effective biostat.

    12. Hi Drew,
      I have some wood cupboards with inside surfaces that are unpainted that smell of mold. Would either of the two formulas work?

    13. I've used formula B before painting many times. Apply and scrub some, wipe off, and let dry, and then prime. Do not rinse; you want to leave a borax residue. I really like white paint inside cupboards, because it creates more light.

      It will not remove all visible mold, but it will kill the smell.