Thick Primer

Q. I am writing with regard to the letter from J. S. concerning the rapid viscosity pick-up of his two- component primer. I believe that temperature also plays a part in his problem. I just assumed that the pressure pot is relatively new, so air leakage is not the problem. We know that an elevated temperature will accelerate the viscosity buildup and curing of the catalyzed material. Some questions I would have asked are: 1) What is the ambient temperature where the pressure pot is placed? 2) What is the temperature of the air entering the pressure pot?

Usually pressure pots have an air-driven agitator to mix the paint. Could high agitation speed be a factor in this viscosity build up?

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Thank you. I enjoy your column. It gives me a chance to recall my happy paint sales days. S.B.

A. S.B. is referring to a question by J.S. in the April Painting Clinic about a two-component primer getting thicker by the minute in a pressure pot. My answer was that the problem was caused by a leaking pressure pot. To maintain pot pressure, more air is flowing into the pot than necessary. This excess air coming into the pressure pot is leaking out, carrying solvent with it. It is well known that removal of solvent increases the rate of reaction of catalyzed paints and decreases the pot life. This phenomenon is a step in the curing mechanism. As more solvent is blown out of the pressure pot through the leak, the reaction rate increases and gellation occurs.

I must have been half asleep when I wrote my answer! (Writing answers for Painting Clinic questions after the 11 p.m. news is a bad thing.) Fortunately, S. B. was wide awake when he read my answer, and he’s right! Increased paint temperature will increase reaction rate, thereby increasing viscosity. To answer his question, high-speed agitation could increase the temperature of the paint and, according to the “Ten Degree Rule,” increase the reaction rate.