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A freshly applied exterior wall coating, and after one rain, white vertical streaks appear all over the wall — this is one of the most common coating defects in the industry. The industry calls it “rain marks,” and internationally it‘s known as “snail trails”. It’s especially noticeable on dark-colored coatings and severely compromises the building‘s appearance.

Rain marks are essentially water-soluble components being washed out of the coating by rainwater. Water-based coatings contain large amounts of surfactants — from emulsions, wetting agents, dispersants, defoamers, thickeners, and color pastes. Most of these components are hydrophilic.

When it rains, water dissolves the hydrophilic components in the film and carries them down the wall. The areas where water flows leave behind shiny or white trails — like giving the wall an “uneven clear coat”.

• Leaching rain marks (shiny spots) — Water-soluble substances in the film are leached out by rain and accumulate on the surface, leaving shiny streaks. Occurs shortly after application (one week to one month) when light rain hits.

• Water-whitening rain marks — Color becomes lighter or white after rain washing, creating streaky color differences.

• Efflorescence rain marks (calcium spots) — Rainwater penetrates the substrate, bringing alkaline substances from cement and lime to the surface, forming white crystals. This is the “efflorescence” phenomenon.

• Dirt rain marks — After prolonged pollutant adsorption, rain washing creates uneven light and dark streaks.

• Choose the right emulsion — Emulsion is the core factor. Soap-free emulsion performs best, silicone-acrylic is second, styrene-acrylic third, pure acrylic worst, elastomeric relatively poor. Among similar emulsions, fewer emulsifiers mean better water resistance and rain mark resistance.

• Choose the right wetting agent and dispersant — Stronger water sensitivity leads to easier rain marks. Choose low water-sensitivity wetting agents with dosage below 0.4%; hydrophobically modified dispersants perform best, followed by ammonium salt types; sodium salt types are worst. Hydrophobic acrylic ammonium salt dispersants are recommended for improved water resistance and rain mark resistance.

• Control color pastes — Iron oxide inorganic color pastes have less impact, organic color pastes have greater impact, carbon black is the worst — keep dosage within 2%; other organic color pastes below 4%.

• Apply clear topcoat — A clear topcoat with excellent water resistance can essentially prevent rain marks. If the clear coat is not fully cured before rain, water penetrates the putty and substrate; when weather clears, moisture brings alkaline substances to the surface.

• Ensure sufficient drying time before rain exposure

• Avoid construction right before or after rain

• Ensure substrate moisture content is below 8%

• Control substrate pH below 10; pH 8 is ideal

• Perform proper waterproofing and fully cure the substrate

• Use high-quality alkali-resistant sealing primer

White streaks on exterior wall coatings after rain ultimately come down to water-soluble components being washed out by rainwater. Remember:

• Choose soap-free or silicone-acrylic emulsions — fewer emulsifiers mean better water resistance

• Use hydrophobic wetting agents and dispersants — stronger water sensitivity means worse rain marks

• Prefer inorganic color pastes — control carbon black and organic paste dosages

• Don‘t skip topcoat and primer — alkali-resistant primer plus waterproof topcoat for double protection