Algae and lichen stains must be removed before exterior house painting because they discolor surfaces and undermine paint adhesion.

Start by testing a small area with a biocidal cleaner following the manufacturer directions and let it sit long enough to kill and loosen growth without overexposing the substrate. For wood and masonry combine one part household bleach with three parts water plus a teaspoon of liquid detergent per gallon to improve wetting when safe for the surface, and always rinse thoroughly after dwell time; for sensitive materials use a commercial labeled biocide designed for exterior use instead of bleach. When using a pressure washer keep the nozzle at least twelve inches off horizontal siding and use low pressure to avoid fiber raising on wood or driving water behind cladding, and always follow with a full rinse to remove dead organisms and cleaning residues. After the surface is dry inspect for hyphal remnants or embedded spores and use a stiff nylon brush to scrub stubborn patches before priming. Apply a mildewcidal primer to all previously stained areas and any porous substrate to block remaining spores and improve topcoat adhesion; select a primer compatible with your chosen topcoat and observe the recommended recoat window. For roofs or repeated algae issues consider zinc or copper strips installed near the ridge to release trace metal ions that inhibit recolonization, and replace siding or shingles that are heavily colonized or structurally compromised because painting over biologically active material shortens coating life.

Three common mistakes are using high pressure that damages substrate and traps moisture, overdiluting or misapplying biocides so organisms are not fully killed, and painting without a mildewcidal primer which lets stains bleed through later. If you use too much pressure back off to a wider spray pattern and finish with a low pressure rinse to protect the material. If the cleaner seems ineffective follow label concentration and contact time or switch to a commercial product made for biological removal. If stains reappear after painting remove the topcoat, retreat the surface and use a primer with mildewcidal properties before repainting.

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