Drawdown charts are rectangular pieces of non-fluorescent paper[1] which are used to test a variety of coating properties.
These properties include opacity, spreading rate, penetration, and flow & leveling behavior.
[2] This non-fluorescent material has to be especially rugged in order to maintain its structure and give reliable readings, as the coatings tested are often corrosive or abrasive.
The bars are generally made of stainless steel or aluminum and touch the chart at each end, while the center is slightly raised to a desired height in order to obtain the desired film thickness.
[4] There are a multiplicity of more complicated designs including some in which the paint is already in the chart apparatus, but the bar method described above is the most commonly used.