Ask the Expert Question-and-Answer Archive

by Ron Joseph

January, 2005

Dry Film Thickness of Inorganic Zinc Rich Primer

Q. Is it true that DFT per coat of Solvent Base Inorganic Zinc shall not exceed 60 microns to maintain the life of paint? We have used Solvent Base Inorganic Zinc paint primer for Heat Recovery System Gas Turbine Stack which will be exposed with a minimum operating temperature of 200 degrees centigrade and a maximum temperature of 285 degrees centigrade. Then we have applied Modified Silicon Acrylic on top of it with a different manufacturer of the primer. The DFT of primer coat is around 150 to 185 microns. While the Modified Silicon Acrylic is around 30 to 50 microns only.

Zinc rich inorganic primers contain very little resin to hold the zinc particles together. Therefore, it is not advisable to apply too thick a coating, else you might loose the cohesion within the coating itself, and the coating might split. Moreover, the coating is not flexible and at high film thickness might crack. This is especially true if the metal surface is heated and cooled freqently, resulting in frequent cycles of expansiion and contraction. The most common range of film thickness for inorganic zinc rich primers is 75-100 microns. By applying 150-160 microns of the zinc rich primer, you are not getting more corrosion resistance nor, in my opinion are you getting better heat resistance. Again, in my opinion you are solely adding to the cost of the primer.

The dry film thickness (DFT) of the modified silicon acrylic seems to be in the correct range, but 30 microns might be a bit too low.

Best wishes,

Ron Joseph