Technical Facts

Temperature

Paint and varnish removers work better in warm temperatures. They are not very effective below 50°F (10°C).

High Humidity

High humidity can affect the way paints dry, especially at temperatures that are below 50°F (10° C). High humidity can interfere with the supply of oxygen necessary for the curing of alkyd paints and can delay the tack-free time from hours to days. With latex paints it is possible for the evaporation of water to be slowed as the paint dries. If the coalescing solvent (film forming glycol) evaporates before the water leaves the film, you will end up with a paint film that is weak or can crack and/or peel.

Corrosive Environment

The more corrosive the environment, the higher the film build of a coating should be to protect a surface. For immersion service, the minimum requirement is 10 mils dry, applied as two or three coats of 3-5 mils each. Interior coatings on walls are usually for appearance only, so 3-4 mils is sufficient. Moderate exposure, such as the exterior of a steel water tank requires 6-8 mils for protection.

Film Thickness

Three coats at 9 mils total film thickness is better than two coats at 10 mils. The three-coat system reduces the chance of misses, pinholes, and solvent entrapment. It is more expensive however, because of labour costs to apply the extra coat.

Painting Steel Surfaces

When painting steel surfaces during periods of cold nights and warm days, it is important to know the dew point temperature and the temperature of the steel. As the temperature and humidity climb during the day, it is possible for the steel to sweat from condensation, much the same as a cold drink taken out of the refrigerator. To guard against painting over condensation, the steel must not be painted if it is within 5°F (3°C) of the dew point. To establish the dew point, a sling psychrometer is required.

Spreading Rates

To establish the theoretical spreading rate in square feet for a liter of paint, you multiply 423.8 times the solids by volume of the product and divide by the recommended dry film thickness. If the solids by volume is 50% and the recommended dry film thickness is 2 mils, the theoretical spreading rate is 423.8 X 0.5 divided by 2 or 105.9 square feet per liter. Theoretical spreading rates do not allow for waste or surface profile (texture).

Temperature

Because water doesn't freeze until the temperature drops to 32°F (0°C), some painters feel that latex paints can be applied down to that temperature. This is not true for all products, because the minimum temperature required for standard latex paint to dry properly is 50°F (10°C). Standard latex paints applied to surfaces below 50°F will form films that are weak and will result in early failure of exterior products and poor washability of interior products.  

In colder weather conditions, use our premium line of Dulux Diamond Exterior products that can be applied at temperatures as low as 36°F (2°C). Check with your local Dulux Paints store to find the product that is right for you and your working conditions. 

Application Problems

Topcoats applied over inorganic zinc tend to pinhole because of the voids in the zinc film. Pinholes can be reduced by using a vinyl wash primer, which is very thin, as an intermediate coat to fill the voids. If vinyl wash is not going to be used, the first coat of finish over the zinc should be reduced 50%, and a light coat applied.

Speeding Up Dry Time

Alkyd paints take longer to dry as the temperature drops. Dry time cannot be speeded up by the addition of a fast evaporating solvent such as Xylol. The fast solvent comes out of the paint quickly but the slower solvents like mineral spirits come out at the same speed they normally would. Japan Driers can speed up dry, but do not exceed the recommended levels on the container — wrinkling may result.

Spray Application

Vinyl and chlorinated rubber paints tend to "cobweb" when sprayed in warm weather. Cobwebbing (the paint comes out of the gun dry like a cobweb) can be reduced by using a warm weather reduction solvent (slower evaporating) or thinning the product with extra xylol.

Spray Application

Dry fog paints are formulated so that the overspray lands as a dry dust after an 8-10-foot fall if the temperature is 70°F (20°C). When the temperature is cool, say 50°F (10°C), the dry is slowed and the overspray particles are dry on the outside and wet in the middle. Pulling equipment or walking through the overspray when conditions are cool can cause the particles to break open and the paint will stick to the floor. When using dry fog in hot weather the dry time is accelerated. Care must be taken that the paint being applied lands wet on the surface being painted, to ensure good adhesion. Dry fog paints dry quickly, and if the paint is partially dry when it hits the surface being painted, poor adhesion will result.

Wood Finishing

If open-grained wood, such as oak or mahogany, is going to be filled with paste wood filler and stained before a clear finish is applied, mix the wood filler and the stain together and combine the staining and filling operations. If the wood is filled before it is stained, the wood filler will not accept the stain the same as the wood and will appear as a lighter color.