The common methods of adhesion testing are performed with a knife, a pull-off adhesion tester or tensile test with fabric. To perform adhesion tests in the field, we recommend the tensile test with fabric. The initial adhesion measured in this test does not mean that long term adhesion is assured but it does provide a strong indication of adhesion performance.
How Much Adhesion is Enough?
The more adhesion the better. Since this is a qualitative test, a frame of reference is valuable, if possible. For instance, measure adhesion of a roof coating to a substrate with and without a primer; or compare two different cleaning methods; or compare two or more different products (perhaps a polyurethane and an acrylic). After any test, determine if the bond failure was adhesive (between coating and substrate) or cohesive (within the coating film).
Field Adhesion Test Procedure
Clean test area and let dry. Do not perform an adhesion test on a rusted surface without rust treatment. Prepare a big enough area to conduct several tests using different recommended coating combinations. Coating alone, coating with primer, primer alone.
Installing the Test Strips
Apply 15 wet mils of roof coating on test surface and embed a 1"x6" swatch of polyester fabric into it. Allow 1" of the fabric to hang free. Completely saturate with another 15 wet mils of roof coating. Allow test patch to dry and cure for at least 3 days. (4 to 7 days recommended)
Doing the Pull Test
Test for roof coating adhesion by pulling the fabric at a 90° angle to the substrate surface. There must be a minimum of two pounds of "pull strength" for the coating adhesion to be considered acceptable. Some roof coating should remain on the substrate.