Remove and Prevent Mildew for Pennies

This guest post is from Drew Frye and first appeared on his blog, Sail Delmarva. Drew also tests products for Practical Sailor magazine and writes for other magazines like Good Old Boat. He graciously agreed to let me reproduce his article here.

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, do 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 acidic conditions. While vinegar has a faithful following, I was able to demonstrate in head-to-head testing that in damp conditions alkaline treatments are more effective.
  • No food. Again, vinegar is a problem because it becomes mildew food when the dampness returns and can 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 that I recommend below, borax is a powerful anti-mold agent and anti-bacterial.
  • Not bleach. While bleach can be effective on the surface, it is damaging to many surfaces and then when it dries, the pH is far too high.

You could troop down to your local home improvement store and pay many dollars per gallon for pennies worth of chemical in a bottle. Plastic, shipping, markup, and paying for know-how all cost. Or you could simply brew up something at home that has proven to be more effective.

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

Formula #1

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 TSP (trisodium phosphate)

This works better than all the commercial formulas on natural fibers and better than 90% of the commercial formulas on synthetics (there are a few specialty formulas based on silicone quaternary 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 leave the final rinse of this treatment to dry in-place. For most cleaning, all that is required is to spray the area until wet, scrub vigorously, and wipe off the excess.

BEFORE – Mildew colonies spreading
AFTER – Six months after treatment

Formula #2

This formula has been a lifesaver on wet basement carpets. The 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.

Borax mildew treatment:

  • 1 quart hot water
  • 2 tablespoons baking soda (sodium bicarbonate)
  • 2 tablespoons borax (sodium borate)
  • 1 tablespoon TSP (trisodium phosphate)

[$tingy note: If you don’t understand why there’s a picture of a 20 mule team with this post, ask your parents.]

By the way, 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 this formula not sold in 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 ingredients, they are exempt from regulation, but the moment you formulate and make claims, the 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 cleaning 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, the most effective anti-mildew cleaner available for pennies. Enjoy.

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24 Comments Add yours

  1. Mike Horoho says:

    Love the mule train reference in the image. Not too many people will get the correlation. Brought to you by Borax!

  2. Rocky says:

    Thank you for your generosity in sharing this valuable information.

  3. Albert Aymer says:

    Thanks for posting this information. Great stuff! By the way,
    I “vaguely” recall the 20-mule team reference!

  4. Ed says:

    So, is this a good treatment for the black stuff that shows up streaking down a roof (or even moss)? I wonder what it does to mosses that grow (in our green NW USA) on stone-work on the house, the underside of decks and the siding in the shade?

    Thanks for the great info!

    1. I don’t know for sure, Ed, but I suppose it would have some effect. Try it out and let us know how it works!

  5. Don Grantham says:

    Thanks for all you do

  6. Richie says:

    Thank for this and your other article. Good stuff.

  7. I made a batch of formula #2 last week, the borax, TSP and spray bottle set me back about $9. It did a good job on some light mold above my shower, and hopefully it will be longer lasting than the diluted bleach I had used previously.

    I’ll take it out to my newly acquired ’73 C22 this week and see how it does on some v-berth headliner mildew.

    I noticed some spots that looked a little funky on the compression post. Is this safe to use on wood?

    1. It should be fine, Eric, but test it on an inconspicuous spot first to be sure you like the results. The formula isn’t very caustic and teak is very resistant to chemicals. I had to treat mine pretty aggressively to remove mildew in Refinish Your Interior Teak to Better Than New and in Restore Your Exterior Teak to Better Than New.

  8. Taylor Fry says:

    So I’ve been cleaning the inside of my new to me boat (sat on the hard for 8 years) with soft scrub with bleach to cut thru it all. Rinsing by rubbing off with wet towels then drying. I wanted to Rejex much of the interior for protection and prevent cleaning, would you put this mixture on before the poly type wax or after?

    1. Hello, Taylor

      I’m not familiar with RegeX but from reading the instructions on their website, it appears to be the last and only product you need after cleaning. For the outside fiberglass, I recommend a good compound buffing and polishing first to restore the gelcoat before applying any protectant.

  9. Marc F says:

    Hello, was curious about the TSP after reading that it is used for prepping before painting, especially oil-based paints. Even though your recipe only calls for 2 tablespoons of TSP, is it ok to use the #2 formula directly on the inner hull area (e.g. under the v-berth)?

    1. Hello, Marc

      Either formula would work anywhere that you have a current or potential mildew problem. Use it to clean as much of the area as you can reach first, to remove possible food for the mildew. Then wipe the area with the formula so that it is saturated but not running or dripping. Use a spray bottle for hard to reach areas.

  10. Sarah says:

    The borax to use, is not the 20 mule team product. I believe you need to purchase actual borax.

    1. You’re probably right but the mules are easier to remember than sodium borate.

  11. Lisa Marker says:

    I’d love to know if, rather than real TSP, a TSP substitute could be used. Thanks!

    1. Hi, Lisa

      That would depend on what the substitute is. You can certainly experiment with whatever you want. This article presents a recipe that is already the result of much experimentation and promotes what was found to work the best.

      Thanks for your question,

  12. Td says:

    Which formula would you recommend for black mold on basement sheetrock?

    1. If it’s just a small area (1 or 2 square feet), I’d just spray straight bleach on it. Any larger, and I’d consider removing and replacing all the affected sheetrock, which might also call far professional remediation. In either case, I’d also be concerned that there’s more lurking behind the sheetrock. Also consider what’s causing it in the first place. If there are foundation or plumbing leaks, fix them. If it’s generally high humidity, increase dry air circulation and/or get a portable dehumidifier.

  13. Jim says:

    I have used your recipe 1 to prep walls before painting, and concrete floors prior to applying a one step epoxy. They both turned out great, and you saved me money!

    I am now curious about spraying recipe 2 into the air intake of window air conditioners. It is a challenge to access the blower wheel and chamber. So, would two spray downs (AC off of course), and full drying time between be effective at reducing Mold and Mildew?


    1. It can’t imagine it wouldn’t help some, at least. I’d even run them outdoors while spraying to help reach those inaccessible areas.


  14. LIsa Hems says:

    TSP is not available in my state. Will TSP – phosphate free still work in recipe 1? Or do you recommend a different substitute, like borax? Thank you for your help.

    1. Hi, Lisa

      I can’t say for sure but it shouldn’t hurt to try. I think the TSP acts mainly as the cleaning agent in these formulas. It might not clean quite as well but maybe well enough for your needs or require a little extra elbow grease or additional treatments.

      Let us know how it works for you,

  15. svrecess says:

    May I ask – why not just take formula 1 and add some borax? I wasn’t sure, so I took your #1 recipe and added the power of 1 tbsp worth of kick ass mules

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