In a perfect world, we’d scrub decks with a green cleaner twice a year and stain, seal, or paint them regularly. Instead, most of us wait a few years before tackling the problem with a commercial deck cleaner, brightener, or stripper. These products work differently, so check the labels for the active ingredients. (Read the safety instructions too, and always keep these products out of the lake.)
Oxygen bleach (sodium percarbonate) is non-toxic and the greenest of the chemical cleaners. When this powder is added to water, it forms hydrogen peroxide, which breaks up dirt and slime, kills mould, and removes the weathered grey layer. It works well to clean mildewed surfaces (bare or painted) without harming the environment. Sodium percarbonate is the active ingredient in OxiClean Versatile Stain Remover, Wash Safe Canada Deck Wash, and others.
Budget choice for mildew
Chlorine bleach (sodium hypochlorite) removes and kills surface mould and lightens the wood. Bleach doesn’t remove dirt, so these products usually include a cleaning agent such as TSP. Chlorine bleach–based products are inexpensive and give similar results as oxygen bleach but may result in uneven coloration and can cause “fuzzing” by breaking down wood fibres. Bleach is not effective against tannin or iron stains. Cleaners with sodium hypochlorite are a good DIY option to clean mildew from bare or painted surfaces—try Olympic Premium Deck Cleaner.
Best for tannins and cedar
Oxalic acid removes stains and kills surface and subsurface mould. This brightener also helps stabilize tannin stains, making it good for cedar decks. One common approach is to use a product with oxygen bleach or chlorine bleach first, followed by oxalic acid, which restores the wood’s natural pH. Oxalic acid is corrosive and harmful to plant life. It’s best used to clean dirty or badly stained decks. Thompson’s WaterSeal Deck Cleaner & Brightener is one oxalic acid–based product that is good try.
Strong and caustic
Sodium hydroxide, a.k.a. lye, in high concentrations can strip paint and sealer from your deck. In low concentrations, it’s found in many cleaning products. It removes mould, most stains, and the wood’s weathered surface layer. Since it is extremely caustic, you must use protective gear. After applying, restore the wood’s pH with a low-pH brightener, such as oxalic acid. You’ll find sodium hydroxide in Thompson’s Water-Seal Maximum Strength Deck Stripper and other products.