WHAT ARE DB'S AND OHMS/SQUARE/MIL?

Since shielding coatings primarily reflect the offending radiation, you would think that it would be expressed as a percentage of radiation reflected, right? No, the measure of shielding effectiveness is expressed in decibels (dB), where:
dB = 10 log10 incident power density
transmitted power density
Got that? If not, maybe it would help to express it as percentages:
20dB = 90% EMI blocked
40db = 99% EMI blocked
60dB = 99.9% EMI blocked
80dB = 99.99% EMI blocked
How about ohms/square/mil? This is a standard way to measure resistance on a flat surface. In order to compare apples to apples in the shielding world, a probe of two blocks (usually 1 Square Inches) separated by the equivalent dimension is used to measure resistance. On a perfectly flat panel with perfectly smooth coating and a user pushing down with exactly the same force from one measurement to the next, the readings will be comparable. This method is used to rough check parts for coating thickness, and is combined with point to point readings of resistance to check parts in production.