Environmental Conditions - Surface Temperatures

Surface and air temperatures are the first conditions to check when assessing the risk of moisture formation on a substrate. The temperature of the surface awaiting preparation or coating application, and the temperature of the air near that surface, can have a great effect on the coatings application itself.

At night, steel usually radiates heat and cools to below air temperature. During the day, it absorbs heat and is usually warmer than the air temperature. Since surface temperature is often different from air temperature, especially for work done outside. Both air and surface temperatures should be measured to avoid application problems should either become too hot or too cold for satisfactory film formation. Application at incorrect temperatures can cause defects such as:

• Blistering

• Pin holing

• Cratering

• Dry spray

• Mud cracking

The coating manufacturer should specify the maximum and minimum surface temperatures for coating application. ASTM D3276, Standard Guide for Painting Inspectors (Metal Substrates), states that the minimum surface temperature for coating application is usually 40°F (5°C). It may be as low as 0°F (-18°C) for cold-curing one or two-component systems or 50°F (10°C) for conventional two-component systems.

Coating specifications may further state that coating should not be undertaken when the temperature is dropping and within 5°F (3°C) of the lower limit. The maximum surface temperature for coating application is typically 125ºF (50ºC), unless otherwise clearly specified.

A surface that is too hot may cause the coating solvents to evaporate so fast that application is difficult, blistering takes place, or a porous film results. Most epoxy formulations stop the curing process below 50°F (10°C). Also of note, we should only apply coatings to a substrate when that substrate is at least 5°F (3°C) above the determined dew point in order to prevent moisture on the surface from being coated.

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