This is a threaded stainless steel rod that functions as a grooved doctor blade at any part of its circumference. It produces a film with parallel ridges and valleys in simulation of brush marks. Critical dimensions are indicated in the diagram. Plastic arms in conjunction with straight-edge guides assure rectilinear drawdown movement. Advantages over brushout application tests are speed, reproducibility, and a regular surface pattern that facilitates evaluation. The leveling of films applied in this manner correlates well with brushout leveling.
The diagram shows alternating clearances of 300 and 100 µm (12 and 4 mils), to apply alternate stripes of 150 and 50 µm (6 and 2 mils) thickness. Thus the mean wet film thickness of the test drawdown will be about 100 µm (4 mils), corresponding to a spreading rate of 10 m²/L (400 ft²/gal).