Yesterday we did a bunch of testing, on several products. (The pictures, and they way I'm presenting it here, is a bit out of order from the way we went through them, so don't compare to stuff far away in the pictures.) All products applied with a hand-pump bleach applicator sprayer (e.g. pressure washer NOT used), or a hand spray bottle.
TLDR: Clorox Outdoor Bleach wins, but everything except Purple Power works well. 3-4 gallons to clean the whole cover, inside & out.
First up...
Purple Power, full concentration, 3 minutes sitting (wet patch above bottle sprayer)
Result: Barely does anything, even after scrubbing.
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Drano Max Gel, full concentration, 3 minutes sitting
Result: Works ok, though slowest of the rest, and was helped by scrubbing. I just wanted to try it for comparison since some of the following products contain Sodium Hydroxide, and had some on hand.
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The Starbrite (far left) and Clorox Outdoor Bleach (far right) are sodium hypochlorite ("bleach") and sodium hydroxide ("Drano") blends. The middle two are just pool bleach (10.5%) and Mold Armor (bleach only).
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Starbrite Mildew Stain Remover, $10/16oz, full concentration, 3 minutes sitting
Result: Worked very well. Dissolved most of the stains within minutes, and rest with a second spray and some scrubbing with a push broom (not shown). Unreasonably expensive compared to the other products, since it's such a small bottle. I won't buy it again, but I refilled the bottle with the Clorox Outdoor Bleach.
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Pinch-A-Penny Liquid Chlorinating Product 10.5% (pool shock bleach), $6/gal, full concentration, 3 minutes sitting
Result: Works well, required a little scrubbing with a push broom
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Mold Armor E-Z Multi-Purpose, $18/gal, full concentration, 3 minutes sitting
Results: Works very well, and wouldn't really require scrubbing. I think it was 5% bleach, making it 1/2 the strength of the pool bleach or Clorox Outdoor. Not sure why it is so expensive... maybe it has special additives for pressure washer use? (The bleach would likely wreck the seals in your pressure washer soap injector anyway.)
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Clorox Outdoor Bleach, $10/gal, full concentration, 1.5 minutes sitting
Result:
Works extremely well... by the time I was done spraying it across the cover panel, the mold was already totally gone. No scrubbing. I don't know why this one worked so well, or fast. It could be that this was the only panel in the sun, which would aid the chemical reaction?
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The Clorox Outdoor Bleach is the clear champion in this totally-not-scientific-test. None of the chemicals harmed the cover, that I can tell. I used 3-4 gallons to do the whole cover, inside and out. I also tried a couple dilution ratios of a few, but they just took longer and required multiple applications, so I personally don't find that to be a money saving approach.
I made a very-diluted mix by swishing ~1/4 bottle of water in each "empty" jug (to rinse them out for recycling), and pouring it back into the sprayer, with some more water. I used that to gently get some green mold off a few spots on the truck paint. It took some time, and a couple applications, but it worked without harming the paint.
Cover looks good as new!
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