The proper test for determining if a paint or finish bonds well to the substrate, which could be just a previous coat of paint or finish, is called the cross-cut adhesion test. It is the standard that has been established by the American Society for Testing and Materials (ASTM).
First, score the coating with the razor blade in a cross-hatch pattern with about one-sixteenth inch between cuts. Then press the masking tape over the cuts and pull it up quickly.The test is shown in the accompanying picture. To be totally accurate, you need a special cross-cut tester (the cutting tool), which is expensive, and special tape. But very close approximations can be achieved with just a razor blade and masking tape.
You judge the bond by the degree to which the little squares stay intact. In the case shown, the bond is not good. Too much of the finish transfers to the tape.
An official scale of ratings can be found on the following website: http://www.allianceorg.com/pdfs/Epoxy_Coat_Test.pdf. But because it’s relative, anyway, you will quickly learn to distinguish between good and bad without the chart.
The logic behind the test is to determine the resistance to separation when a coating is struck by a blunt object or has a coarse object dragged across it under pressure. You can imagine a child doing either of these.
Therefore, two instructions that I’ve seen recently for the test are not correct.
One puts the tape on the surface first, then makes the cuts through the tape. The other simply applies tape and pulls it up—no cuts. Neither provides accurate results. In fact, both would almost surely indicate perfect bonding, even if it were, in fact, really poor.