Wet Film Preparation

Reliable and accurate testing of coatings and films, such as those in the paint and coatings industry, requires rigid control of their actual (not assumed) wet and dry film thickness. Hiding power, opaqueness, scrub resistance, sag resistance, and many other quality tests require a uniform, repeatable film for routine QC and R&D testing. Many ASTM, ISO, Federal, and other specifications specify films of predetermined and uniform thickness.
The most common method of spreading a sample is with a film applicator, also called a drawdown bar or doctor blade. Film applicators can lay down wet films of almost any desired thickness from 0.1 mil up to more than 40 mils. One mil is equal to 1/1000 of an inch, or 25.4 microns (µm).
A typical blade type applicator consists of a metal bar containing a gap of known depth or clearance on one or more faces. It is placed near one end of a flat panel, such as drawdown chart . A sufficient volume of sample is placed in front of the applicator. The applicator is then “drawn down” the chart, either mechanically or manually, leaving a uniform film behind it. Mechanical methods give better repeatability and accuracy, as operator deviations are minimized.

Stainless steel, aluminum, or plated steel are the preferred materials of construction, due to their resistance to corrosion. Plated and stainless steel types are harder and will withstand more rigorous use. Regardless of the material of construction, corrosion can damage the region of the drawdown bar controlling thickness of the applied film, therefore affecting the repeatability of the instrument. Good lab practices dictate immediate cleaning of the instrument after each use to eliminate potential corrosion or residue which could affect future results.