Why Test for Coating Adhesion?
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 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 thoroughly and let dry. Do not perform an adhesion test on a rusted surface without rust treatment or metal primer.
- Apply 20 wet mils of roof coating on the prepared or primed surface and embed a 1"x6" swatch of polyester fabric into the roof coating. Allow 2" of the fabric to hang free.
- Apply another 20 wet mils of roof coating on the fabric until it is completely saturated. Allow roof coating to dry and cure for at least 3 days. (10 to 14 days recommended to avoid false positives.)
- Test for coating adhesion by pulling the fabric at a 90° angle to the substrate surface. There must be a minimum of four pounds of "pull strength" for the coating adhesion to be considered acceptable. Some roof coating should remain on the substrate (see picture).
- Conduct several tests using different recommended coating combinations. Coating alone, coating with primer, primer alone. Use a test strip 12" or longer. After 7 days, pull about 3". At 14 days another 3" and another one at 30 days.