Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

September, 2003

Specified Paint Thickness

Q. We have specified in our technical specification a transformer tank as follows:

All steel surfaces to weather shall be givend a primary coat of zinc chromate, second coat of oil and weather resistant varnish of a color distinct from primary and final two coats of glossy oil and weather resisting non-fading paint of color light grey corresponding to shade RAL5012.

  • Primary coat of Zinc Chromate - 70 micron 4 hours
  • Secondary coat of Varsnish - 70 micron 4 hours
  • Under coat (first coat) Paint - 70 micron 8 hours
  • Final Paint - 70 micron 6 hours

I would like to know the final thickness of paint?

A. Are you sure you want to apply such heavy coats of paint? You are specifying 70 microns (2.8 mils) per coat for a total of 280 microns (11.2 mils). This would be fine for some heavy build epoxy coatings, but for "oil & weather resistant varnish" it is excessive. First, I am not aware that you can apply one coat of any of these coatings to give you 70 microns. In my opinion it will take at least two coats of each to get to this film thickness. But even if you can achieve this for each coat of paint, you run the risk of having severe blistering.

Can I assume that the hours stated next to each item is the intercoating time? Are you requiring that the zinc chromate primer be overcoated within 4 hours? If so, you are not allowing sufficient time for the solvent to evaporate and you can expect major paint failures in the form of peeling. How will the paint be applied, by brush or spray?

Before I can advise you further, you will need to tell me where the transformers are to be exposed (type of climates) and the surface preparation prior to painting.

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