How to measure coating hardness using the Wolff Wilborn Pencil Hardness Test
If a coating does not dry (or cure) properly, this can affect its performance; resulting in flaws, poor adhesion, and premature coating failures.
This is why we test coating hardness, as this indicates how well it has cured – typically the harder the coating, the more complete the cure, and the better its overall performance.
One of the most common methods of determining coating hardness is the pencil hardness test, also known as the Wolff-Wilborn method, where a pencil of a known hardness is pushed across the coating at a specified angle, under a constant force. The pencil hardness is increased from B (soft), though HB (medium) to H (hard), until the coating is scratched. Alternatively, depending on the test method, you may start with the hardest pencil, and then decrease the pencil hardness until the coating isn’t scratched.
This rapid, inexpensive method is not only used by coating manufacturers during developmental work and as an indication of the coating’s performance; but also as a field test during post-application inspection.
The pencil hardness test can be completed free-hand using the Elcometer 3080, which includes a set of 14 pencils (grades 6H to 6B), two specialised pencil sharpeners, and an abrasive paper block to prepare the pencil for testing. Alternatively, some Standards state you should use a specially designed tester, such as the Elcometer 501; which also includes a set of 14 pencils (again, 6H to 6B), the two specialised sharpeners and abrasive paper block, and a setting block of a predefined size for preparing the tester.
Remember, the scratch method is a destructive test and will damage the coating, so we recommend using test panels when testing using this method.