This article, written by John Harris, was published in the August 1993 issue of The Paint Dealer. John had developed a small, reuseable self-adhesive thermometer that can be used to measure and indicate the temperature of any surface before painting. The Paint Dealer was kind enough to publish a press release of this product, and asked John if he would write an article that would help explain why it important to paint a surface within the proper temperature range.

Technician's Notebook:

Helping Painters Work At A Proper Substrate Temperature

Our intent with the introduction of temPAINTure is not only to provide paint stores with a much needed tool, but to help customers understand the importance of surface temperature and how it relates to the longevity of the paint products they use.

Dedicated paint chemists spend a lot of time and research dollars selecting and balancing the proper raw materials which provide us with the best coatings possible.

Since you cannot see these efforts when you pop the lid and look into a can of paint, much of the value of this technology must be experienced during the application and by the durability of the product.

Paint manufacturers ask that we observe a few precautions when using their products, and for a very good reason; unless we do, the paint just won't perform the way it was intended.

One precaution most manufacturers ask is that we apply paint to a surface that is within a safe temperature range. The damage that a cold surface (below 50 degrees Fahrenheit) or hot surface (above 90 degrees Fahrenheit) can do to most paint products is considerable. Painting a surface outside the safe temperature range will produce a result which will be disappointing to the customer, the painting contractor, the paint store employees, and the paint manufacturer.

To understand the importance of the role of surface temperature, it may be helpful to discuss some of the dynamics inside the wet paint film.

Latex paint is a mixture of pigments, water, latex, coalescing agents, and several other raw materials. Under recommended drying conditions (50 to 90 degrees F), the water evaporates first. This results in reduction of the wet film thickness, bring the latex resin particles close enough to touch one another. The coalescing agent is a solvent which evaporates slower than water. The function of the coalescing agent is to remain in the film after the water evaporates and to soften the latex resin particles enough so that the particles join (or coalesce), creating a continuous film. To preform properly, this film must stick to the substrate, bind the pigment, and generally give the customer a result worthy of the investment made in the product and the labor of application.

If a paint is applied to a surface which is too cold, the latex resin particles become too hard for the coalescing agent to assist in proper film formation. If the coalescing agent evaporates without doing its job, permanent damage to the film results. The particles of resin do not properly stick to each other, to the surface, or to the pigment. The resulting coating may exhibit defects such as inadequate adhesion, water streaking, cracking, crumbling and poor weather-ability.

If the temperature of the surface is too hot when the paint is applied, the coalescing agent may evaporate too quickly, again resulting in problems with the final paint film formation, such as poor adhesion and poor leveling, blistering, extreme lapping, and poor weather-ability.

Oil base paint is less sensitive to surface temperature extremes; however, 50 to 90 degrees F is still optimum. Colder temperatures retard the drying process of oil paints, allowing more time for dirt to stick on the wet film. Surfaces which are too warm will not allow the oil paint to properly flow and level. Above 90 degrees F, oil paints may prematurely surface skin, trapping some of the solvent within the film. This is one causes of blistering; when trapped solvents that cannot evaporate through a surface skin, become warm enough to vaporize, gas bubbles or blisters form inside the coating.

Many of these problems are not apparent immediately. Much of the success or failure of a coat of paint cannot initially be seen by the naked eye. It may take time for a problem to develop into a visible film defect. An additional disappointment is that another properly applied coat of paint will not repair the damage done to a previous coat that has been applied at the wrong temperature.

Conscientious paint store personnel may advise customers of this information but the painter is on his own when it is time to actually apply the paint. Air temperature has been inaccurately used by painters a guide to determine when it is safe to paint. The inaccuracies, of course, lie in facts such as: 1) air temperature will change more rapidly than the temperature of a concrete wall, and 2) the radiant energy of the sun will increase the temperature of a dark surface faster and high than air temperature. I have heard some painters say, "I have a crew on location. The paint is going on regardless of the surface temperature." I reply, "That's fine, just don't blame me or the paint manufacturer if there are coating failures later."

My TemPAINTure surface temperature thermometer will give you the answer when you hear, "How do I KNOW when the surface temperature is safe to paint?"

If you would like to purchase one of these surface temperature thermometers, let me know. They cost $2.99 and fit in a business envelope. I can mail one to you today.