Tuesday, March 17, 2015

Out Damned Spot!

This is a mash-up of posts which originally appeared on Windborne in Puget Sound

On Eolian's decks hundreds of black spots have been slowly appearing, and have been doing so ever since we repainted the decks.  They don't grow fast, but they do grow.  Nothing seems to touch them - using straight bleach has no more effect than plain water.

Supposing that these are some form of lichen, I did some research - it is such a wonderful thing to have the bulk of Man's knowledge at your fingertips!
Guess what?  Algae, moss and lichens suffer grave bodily harm when exposed to a particular class of chemicals.  Well I guess that part is not a surprise.  But this part is:  that family of chemicals is relatively harmless to virtually everything else.  In fact, it is the active ingredient in Bactine:  benzalkonium chlorides.

Now here's the next surprise.  No, you don't have to buy 100 bottles of Bactine and distill it to get the benzalkonium chloride - all you have to do is go to the pool/spa section of your local hardware store and buy a bottle of HTH Algae Guard:

And it was less than $10!

This is a 30% solution, and yet a 2% solution is supposed to be adequate for killing algae/moss/lichen.  

So, the first test is a kind Hippocratic one.  Does this stuff harm the Sunbrella canvas on the boat?  I uncapped the bottle and put some of the straight 30% solution directly on a scrap of our Sunbrella and left it to dry.  After rinsing it out 24 hours later, there was no detectable effect on the canvas.  At 30%, the solution has a blue tint - I suspect this is just a dye for appearances, given its intended use. Our canvas is green - if yours is white, you might want to repeat this test.

For the next test, I made up a 6:1 dilution (5%) solution in a hand spray bottle and applied it to a section of the deck with the lichen, and a portion of our canvas which has a liberal infestation of winter algae on the outside.  I also applied it to the inside of the canvas directly over our galley vent where we get the most amazing colonies of...  well, life I guess, apparently feeding on whatever the vent delivers to the canvas.


The benzalkonium chlorides definitely killed the lichen.  But it did take a mild scrubbing with a brush to remove the corpses.  Please note that previous scrubbing with the same brush and bleach had no effect on the spots.  I apologize that I used a washer for the sizing comparison - I didn't have a dime in my pocket.  The washer is a little bigger than a dime and a little smaller than a nickle.

I also applied the 5% benzalkonium chloride solution to a portion of our dodger canvas.  Please note that this canvas is more than 10 years old, and has lost most of its water repellant qualities - that's why the mold/mildew can live in it.


The "After" photograph does not do the results justice.  The mold, mildew and algae are all dead.  A couple of rainstorms washed the bulk of the corpses away, except at the seams.  But more rain is coming (of course).


This stuff works!  And Jane tried it on the moss on our driveway at a 3% concentration, as another experiment.  It worked there too - the moss is all dead.  In fact, using a 5% solution was probably overkill - my next application on Eolian's canvas will be at 3%, but I think I'll retain the 5% level for the tough-to-kill lichen on the decks.


  1. Thank you! Algae guard for swimming pools, which does not contain chlorine! I have previously sprayed chlorox on my sunbrella, waited 20 minutes then washed it off for another 10 minutes (lots of water out of worry that the chlorox would go after then Sunbrella). Thanks again!

  2. Thanks Robert. This worked great on our sunbrella. I didn't work the crevices too hard and will have another go at them with a smaller brush, but no noticeable discolouration (dark blue, pieces ranging from 3-7 years old).

  3. The algae guard that I ended up with looks a bit different than yours - it doesn't say 3x concentrate so I'm guessing it's 10%? Two types of benzalkonium are listed for an active ingredient total of 10% with 90% inactive ingredients. DOes that sound about right to you? From your article I'm guessing I need a 3:1 dilution for decks and canvas. Thanks. Dana

  4. Hi Dana -
    Well, a 2:1 (2 parts water to 1 part concentrate) dilution would give about 3% which would be about right. This is not an exact science!



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