The outdoors


Exterior products should be able to withstand more extreme environmental conditions than those inside.

Exterior products should be able to withstand more extreme environmental conditions than those inside. The main enemies for exterior products are UV rays, moisture and temperature fluctuations.

Sunlight breaks down binders used in coatings. When a binder decomposes, chalking occurs and paint pigments are released in powder form. As these pigments are washed away by rain, the paint colour loses its lustre and the protection once provided by the coating diminishes. Those surfaces most exposed to the sun are located on the southern and western faces of a building.

Moisture weakens paint coatings, causing swelling, cracking and peeling. Moisture evaporates quickly in sunlight, but slowly in shade. Lingering moisture causes the formation of mildew. Shaded surfaces are ideal spots for mildew to develop.


Temperature fluctuations cause exterior surfaces to expand and contract. If the coating has lost its flex, it may crack or peel. Exterior coating that has been applied under extreme temperatures may not adhere properly. The ideal temperature for applying water and solvent-based coatings is between 15 °C and 25 °C (60 °F and 77 °F).

Make sure the surface to be painted is the same temperature as the surrounding air. In other words, do not paint in direct sunlight. If the wall you paint is too hot, the water or thinner in the coating will evaporate before its binder can adhere to the surface. If the wall is too cold, the water or thinner will be slow to evaporate, delaying drying with solvent-based coatings and reducing coalescence with water-based coatings.

During painting, wind can hasten the evaporation of thinner or water, drying the coating. Overly rapid drying may reduce adhesion of coating. Wind can also blow dirt and dust onto wet coatings, reducing adhesion and tarnishing appearance.

The three main enemies of exterior coatings



Temperature fluctuations

Manufacturers fine-tune their product formulas to respond to such problems. Products designed for outdoor use contain more resins, more flexible resins and more adhesives than products used indoors. This increases their flexibility and elasticity and means they can better withstand changes in temperature and humidity. They also contain special additives for resisting water, mildew and sunlight.