You must remove all visible mold and algae before sealing so trapped growth does not cause failure.

Start by identifying affected surfaces and testing a small patch for cleaning compatibility, because porous materials like untreated wood and textured concrete need gentler approaches than painted drywall. Use a diluted bleach solution of one part household bleach to ten parts water or a commercial mold cleaner, and apply with a stiff brush while wearing gloves, eye protection, and an appropriate respirator for enclosed spaces. Agitate thoroughly until staining and growth lift, then rinse the area with clean water using a sponge or low pressure hose to remove residue; do not leave cleaning solution to dry on the surface. For porous substrates that retain moisture, follow with blotting using clean towels and increase ventilation with fans or dehumidifiers until the material is completely dry, which can take twenty four to seventy two hours depending on humidity. If the substrate is soft or crumbling, remove and replace that section rather than attempting to seal over it, because sealants will not adhere to unstable material. After drying, inspect for hidden growth in joints or behind trim and repeat cleaning for any remaining colonies, then proceed with sealing only when the surface is fully dry and free of odor or visible spots.

Common mistakes start with using undiluted bleach on every material which can damage paint and wood so always test a small area and dilute properly. People often skip thorough rinsing and drying which leaves residue and moisture that trap spores beneath a sealant so rinse until foam is gone and dry completely. Another frequent issue is neglecting ventilation and protective gear which risks health and slows drying, so use fans, dehumidifiers, and wear gloves and a respirator rated for mold particulates.

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