Tannin
Tannins are natural plant polyphenols — the same compounds that give red wine, tea, and oak bark their characteristic astringent flavor. In exterior cleaning, they are the source of the persistent brown, red, and orange streaking that runs down siding beneath overhanging trees (especially oak, maple, walnut, and cedar) and the rusty shadows that form under wood decks and pergolas. Acorn drip, leaf accumulation, and cedar-shake runoff are classic tannin sources.
In depth
Tannin stains are tricky because sodium hypochlorite, the workhorse of soft washing, can actually set them into the surface permanently if applied without the right pre-treatment. The correct approach is an acidic pre-treatment (oxalic acid or a tannin-specific brightener) to dissolve and lift the stain, followed by a standard soft wash. On painted surfaces and wood, this two-step process is the only reliable way to get the stain to release without damage.
How this shows up on our jobs
Every spring in Madison we treat dozens of homes with tannin streaks from last fall's leaf accumulation. We always pre-treat with a brightener before the soft wash — otherwise the bleach sets the stain.
Services where this matters
Related terms
Oxidation
Oxidation is the chemical reaction between a surface and oxygen (plus UV light, moisture, and time) that produces a chalky, faded, or dulled appearance. On vinyl siding, oxidation shows up as a powdery white residue that rubs off on your hand. On fiberglass doors and garage doors, it is a milky film that makes the surface look faded. On aluminum gutters, it appears as a crusty, chalk-like coating called "tiger striping" when rain streaks it. On painted metal, oxidation is the precursor to true rust.
Organic vs. Inorganic Stain
Organic stains are caused by living or once-living things: algae, mold, mildew, lichen, tannins from leaves, berry drip, insect excretion, pet urine, and so on. They contain carbon and respond to oxidizing chemistry — sodium hypochlorite or hydrogen peroxide (percarbonate) — which breaks the carbon bonds and destroys the stain at the molecular level. Inorganic stains are mineral-based: rust, hard-water deposits, efflorescence, metallic irrigation over-spray, fertilizer streaks, and the like. They contain no carbon and will not respond to bleach at all — you need an acid or a chelating agent to dissolve the metal or mineral and carry it away.
Pollen & Honeydew
Pollen and honeydew are two separate sticky contaminants that are constantly blamed on each other. Pollen is the yellow dust that coats Wisconsin cars, siding, and windows every spring from late April to early June — primarily oak, maple, birch, and grass pollens. Honeydew is the clear, sticky, sugary fluid excreted by aphids and scale insects that feed in the canopies of maples, lindens, and elms during summer. Honeydew dries into a lacquer-like film that attracts sooty mold and quickly turns black.
Percarbonate
Sodium percarbonate is a solid granular compound that releases hydrogen peroxide and soda ash when dissolved in warm water — essentially an oxygen-based bleach. Unlike chlorine bleach, percarbonate does not contain chlorine, does not off-gas toxic fumes, and breaks down into water, oxygen, and soda ash after use. It is the primary chemistry behind most "wood-safe" deck, fence, and cedar-shake cleaners on the market.
Acid Wash
An acid wash in exterior cleaning is the application of a dilute acidic solution — most commonly muriatic (hydrochloric), phosphoric, oxalic, or a proprietary masonry blend — to dissolve mineral-based staining that bleach and surfactants cannot touch. Typical targets include efflorescence on brick, heavy rust from fertilizer or irrigation, mortar haze on new brickwork, mineral deposits on glass, and battery acid on concrete. Acid wash is not a general cleaning method — it is a targeted chemistry tool for specific stains on specific substrates.