Adhesion testers are used to test how well a coating has adhered to a surface, as poor adhesion can lead to
premature coating failures. The coating inspector has essentially three main methods to choose from - cross
hatch, push off, and the most common, the pull-off adhesion test.
When it comes to selecting the dolly diameter and adhesion gauge type to use for a pull-off adhesion test;
factors such as the substrate material, the substrate shape, and the expected coating strength will affect the
dolly or gauge the user chooses.
When preparing a pull-off adhesion test, whilst the actual location and the number of pulls is typically defined
by the test method or procedure that’s being followed, the actual positioning of the dolly is vital, as poor
positions can result in low pull-off values or invalid tests.
Properly preparing the dolly and the surface before performing a pull-off adhesion test will help achieve
higher pull-off values.
Should the coating around a dolly be cut, or left uncut prior to a pull-off adhesion test; and whose decision is
it to make?
Once the glue between a properly prepared dolly or pull stub and a properly prepared coating has fully
cured, the pull-off adhesion test can be undertaken using the adhesion tester of choice.
Once the pull-off adhesion test has been completed the tensile pull-off force, whether the coating was coat or
not, the dolly size and gauge type, and the failure mode of the coating are all recorded; with the nature of the
failure usually recorded as an estimated percentage to the nearest 10%.
Testing coating adhesion on concrete substrates using a pull-off adhesion gauge is similar to testing coatings
on metal substrates. However, there are a number of differences in the test procedures and equipment used.