Simple Adhesion Testing

By Jef McCurdy

Coatings manufacturers mean well, but don’t always make recommendations that are practical in the real world. Some of the processes they prescribe are great on paper, written by engineers in a laboratory. One glaring example of this is adhesion testing. Most manufacturers recommend pre-cleaning, pressure washing, and drying before setting the test, waiting 3-7 days, and performing adhesion testing.

While this is a good process for preparing a whole project for most coatings, it is beyond impractical for simply determining if a substrate is a good candidate for a particular roof coating. Coatings contractors need to know a lot during the bidding process. Is the substrate harboring moisture? Is there remedial work that needs to be done before a project can be coated? Will the proposed coating adhere to the substrate properly?

Moisture meters and proper project walks can determine the first two questions, but how do you determine the latter in a real-world situation? First off, here are two things I absolutely do not want to do: (A) pressure wash a project that has not been sold yet, and (B) find out after the job is sold that the product will not adhere properly. What would you tell the client in that scenario?

My method for adhesion testing is simple and requires only a few items that are easily carried with you while you perform your roof walk. Here is a list:

  • Small wire brush
  • Can of compressed air or light brush for dust
  • Chip brush
  • 6” long by 2” wide strip of polyester seam tape
  • Small sample of the product to be tested

The process is simple: Scrub an oversized section with the wire brush, and blow or brush the dust away. Apply enough product to imbed ½ the length of the strip (for silicone 15 mils is good). Using the brush, make sure there are no air pockets. This step is important. Air pockets can give you a false reading. Apply a thin layer of product over the test strip.

If this is a fast cure product or accelerators are available, my test strips will be ready to check before I have finished measuring and walking the roof. Otherwise, I will schedule to come back in a couple days. The honest truth for most products is that they will either pass or fail the next morning. Allowing 7 days to achieve full cure typically does not alter these results. 

This method will not work in greasy or severely soiled areas, but is good for the majority of projects you may be bidding. It is also not intended to be a substitute to ASTM testing for evaluating coating properties. However, it will be a quick and easy way for you to ensure that a coating adheres to the prospective substrate.

Evaluating Results

The examples above were tested on new torch down.

Fail: The majority of the product separates from the substrate with little to no resistance. You will notice that very few of the granules were disturbed.

Good: A majority of the product remains on the substrate, with some granule disturbance.

Better: About 50% of the product remains, while there is significant substrate damage.

Best: The product clearly out performed the substrate, leaving a bare spot where the test was performed.

Note: These methods are non-scientific and not intended to be a substitute for common sense.

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