10.2. Paint Film Thickness

10.2.1. How is paint film thickness determined?

The most commonly used paint-film-thickness measuring devices are wet-film thickness gauges and magnetic instruments for dry-film thickness measurement. When properly used during paint application, a wet film gauge is a direct-reading instrument that furnishes an immediate indication of thickness at a time when inadequacies can be corrected, usually without the need for a full subsequent coat. The residual dry-film thickness can be determined from the wet-film thickness because the percent volume of solids in most paints is known. Alternatively, the correlation can be determined from actual dry-film thickness measurements taken at several areas. The readings of magnetic instruments for measurement of dry film thickness are often misinterpreted because they depend upon a number of variables such as initial calibration, type of cleaning, blast pattern profile, amount of mill scale remaining, and relative hardness of the paint film. However, when properly used, both wet-film and dry-film measurements provide an indication of the thickness of the paint over the peaks of the surface profile. 

The primary measuring device for most types of paint should be the wet-film thickness gauge used during actual painting, with proper correlation to the percent volume of solids in the paint being applied. When magnetic instruments are used as a check on the dry film, SSPC-PA2 should be used for the dry-film thickness measurement.

last modified 18 September 2002

10.2.2. What frequency of paint film thickness inspection is appropriate?

A sampling plan is defined in SSPC-A2 on the basis of the square footage of the structure being painted, which is useful for field painting applications. For sampling in shop painting applications, AISC recommends that 2 members be tested in every 25 tons or each shop layout of pieces to be painted. Any deficiencies in paint thickness or other specification requirements must be called to the attention of the fabricator by the owner/inspector at the time of completion of painting.

last modified 18 September 2002

10.2.3. Is a thicker paint film thickness than required acceptable?

Yes. Because the specified paint thickness is usually a minimum requirement, greater thickness is permitted if it does not cause excessive mud cracking, runs, sags, or other defects of appearance or function.

last modified 18 September 2002