Exterior Surface Finishing
Exterior Surface Finishing tests coating performance under weather, substrate movement, and contamination. The exam expects substrate-specific preparation, dew-point control, and recognition of chalk, mildew, and surfactant leaching.
Substrate preparation
- Wood should be dry, commonly 12–15% moisture content, with knots sealed and end grain back-primed. Replace rotted wood rather than coating over it.
- Masonry must cure, typically 28 days, and be at pH 10 or below with low moisture before coating. Alkyds on fresh alkaline masonry can saponify.
- Steel needs rust and mill scale removed to the specified SSPC or ISO grade, then a compatible primer. Galvanized metal needs weathering or an etch, plus a zinc-phosphate or suitable primer.
Weather and application
- Apply above the minimum substrate and air temperature, with surface temperature at least 3°C above dew point, and rising.
- Avoid direct hot sun, high wind, and imminent rain. Surfactant leaching appears as glossy runs when waterborne coatings are applied in cool, humid conditions.
- Treat mildew with an appropriate wash, rinse thoroughly, and use a mildew-resistant coating where appropriate.
Worked example
Air is 20°C at 60% relative humidity, giving a dew point near 12°C. With a 3°C safety margin, the surface should be at least 15°C. If the wall reads 10°C, painting should stop because moisture can condense on the surface before the film forms.
Traps that cost marks
Painting over chalk without cleaning and a bonding primer; applying alkyd to damp masonry; ignoring dew point because the air feels warm; failing to back-prime wood; and washing mildew without rinsing the biocide residue.
Key points
- Match preparation to wood, masonry, steel, and galvanized metal.
- Check moisture and pH before exterior coating.
- Keep surface temperature at least 3°C above dew point.
- Remove chalk and mildew; rinse residues.
- Use primers and back-priming to control moisture and adhesion.