Proper Wet Tape Adhesion Testing Procedure
What are proper procedures and evaluations for meeting wet tape test requirements common to the aerospace industry?
Q. Our paint team is currently having trouble meeting wet tape test requirements. The primer is an aerospace certified product and consistently fails a wet tape test. What advice can you offer us on the proper procedures and evaluations?
A. With out knowing which ASTM protocol you are using it would be difficult to answer your question 100 percent. One of the standards typically used for this type of industry is ASTM D3359, designed to evaluate mechanical adhesion of a coating to a metal substrate.
If you have not already, you will want to purchase the proper ASTM testing device, which typically consist of a 4.5-pound roller. The device is 3.5 inches wide with a Durometer hardness ranging from 70-80. They are made of steel and have two plastic handles. One note: The roller device should be stored on its side on the stand that is usually provided with the device. This is important to ensure the roller stays round, as improper storage can potentially flatten the surface. You will also want to purchase the proper carbide metal scriber tool outfitted with a 1/16-inch tip used to cut the painted surface for testing.
Start the process by following your coating manufacturer’s recommendations for proper surface preparation before the coating application and proper mil thickness application, and dry/cure times. For instance, if the testing criteria is designed for 5 mils of paint application, and you only apply 2-3 mils, you could potentially alter a favorable testing result; conversely, if you apply 8, 9 or 10 mils, you could potentially create a more favorable test result. Following the proper protocol is extremely important for accurate testing results.
Once the coating has cured to the coating manufacturer’s specifications, submerge the testing coupon/specimen in clean distilled water for 24 hours.
After the 24-hour immersion period, remove the testing piece from the water and thoroughly dry the surface. You will now need to make two parallel scratches one inch apart using the carbide cutting tool and firmly cutting all the way to the metal surface within one minute of drying the part. I generally make the cuts 3-5 inches in length. Next, you would apply the proper one-inch ASTM specified 3M No. 250 masking tape immediately at a right angle to the parallel cuts, (approx. 4-5 inches in length) and pass the 4.5-pound roller over the length of the tape back and forth 8 times. Last, angle the tape at approximately 45 degrees, then firmly holding the end of the tape give it a fast pull off the surface. I usually test 2-3 surface areas on the specimen to determine an overall average. Generally the pass/fail is graded on a progressive scale. The more paint that is removed, the worse the overall grade would be.
One important note is that the tape generally only has a shelf life of six months at room temperature. Beyond this, the tape could potentially not be effective for testing. You can also reference ASTM D3330 for peel adhesion of pressure-sensitive tape to validate tape effectiveness.
Black anodize finishes have long been favored for their thick, hard coating properties. It is also these very same characteristics that often make the anodizing unusable for many military and aerospace applications.
This system improves buffability and extends service life.
The following anodizing process overviews are provided as a means of introduction to aerospace anodizing