Primer Getting Thick

Q. I am the manager of finishing operations in our plant. My paint line foreman tells me that the two-component primer we are using is getting thicker by the minute. Because the pot life was long enough, we had been mixing up a five-gallon batch of primer every day and applying it using siphon-feed spray guns on our products.

We did this for many years without any problems. The thickening problem started after we started using a five-gallon pressure pot to feed our spray guns. Now the primer gets so thick that it clogs the spray guns. Adding solvent doesn’t help. When we go back to siphon-feed spray guns, the problem goes away and we can mix up a five-gallon batch that lasts for two shifts. We went to the five-gallon pressure pot to eliminate refilling the small siphon cups. Our production has increased to where it is more convenient to use the five-gallon pressure pots. What can we do? J. S.

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A. My guess is that your problem is 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. Do this long enough and you will have a solid casting in the pressure pot. I suggest you check the pressure pot lid gasket, agitator shaft seal, and pressure relief valve for air leaks. Stopping the leak should stop the problem.