Alkyds as coatings

Citation

(2001), "Alkyds as coatings", Anti-Corrosion Methods and Materials, Vol. 48 No. 5. https://doi.org/10.1108/acmm.2001.12848eaf.003

Alkyds as coatings

Alkyds as coatings

Keywords: Coatings, Alkyds

Compatibility is one reason why alkyds remain a preferred coating. With proper care, they can be applied over a wide variety of existing coatings.

Old alkyds need only be cleaned of contaminants, as they normally lose enough gloss after the first year to present a good surface for adhesion. Acrylics need to have the gloss reduced by sanding with 80-120 grit sandpaper. Epoxies need to have the chalky removed by power washing. If there is any gloss left, that gloss must be broken by either brush blasting or sanding with 80-120 grit sandpaper. Polyurethanes are more likely to retain a glossy finish that must be abraded with brush blasting or sanding to allow adhesion with an alkyd.

Alkyds can also be applied over newly applied epoxies, provided the alkyd finish is applied while the epoxy is still relatively soft, in its early stages of cure. In this case, the adhesion is quite good. If the new epoxy cures hard and glossy, alkyds will not adhere to it well.

The surfaces which alkyds will fail are those that are caustic (basic), with a pH of more than nine. These include concrete, gypsum, IOZ, and galvanized metal.

Resistance is a weak point for most alkyds. Because there is no chemical reaction between two components, the cured film has less density than either epoxy or polyurethane. This makes it more susceptible to penetration by solvents and chemicals. Normal weathering from rain and sun will eventually destroy the alkyd's gloss but it usually does not destroy the integrity of a properly cured coating.

Alkyds should not be used in immersion. They have poor water, chemical and solvent resistance during constant immersion. Some alkyds, such as tung oil, are recommended for marine applications but usually not in immersion areas.

Despite the effects of US government regulations restricting the amount of solvent emissions to the atmosphere, alkyds remain a popular choice for many atmospheric services in both marine and industrial environments. Newer resins have increased the volume solids range from 1,950s and 1,960s, which meets most volatile organic compound restrictions. While some users have switched to water based acrylics to lower the overall amount of solvent emissions from their facilities, the water based acrylics cannot be applied or cured at temperatures much below 40°F. Any water that freezes in the applied film will destroy the integrity of the coating. They also provide good corrosion resistance.