A simple but effective way to double-check multi-coating finishes.
Today, we see many different coatings being used in the same finishing processes, and I believe we need to be very careful that we still are getting good adhesion between the substrate and all the intermediates that are used to make up the entire finish. These include any sealer, stain, toner, glaze, shading stain and the clear coatings. Let’s look at some examples of common finishes that combine at least two coatings.
Some finishers like to use de-waxed shellac under their water-based coatings. The reason this is done is to seal off the wood. De-waxed shellac also is used to prevent the wood’s grain from rising, because some water-based coatings raise the wood grain when used alone.
Another two-coating method is to use de-waxed shellac as a barrier coating. This is an old technique generally used to seal in any kind of oily residue, including silicone contamination that could cause serious “fish-eye” problems. Once the de-waxed shellac is dry, the finisher applies other clear coatings to complete the work.
Following are two other examples of combination coatings. Vinyl sealers are intended as barrier coats to seal wood from excessive moisture. After the vinyl sealer has dried, it is clearcoated with other coatings, such as conversion varnish or a pre- or post-catalyzed coating, to complete the two-coat finish. Another widespread use for vinyl sealer is to apply it as a barrier coating to seal in colored glazes and then topcoat it with a dissimilar coating.
All of these coating combinations seem to work, but how good is their bond to the substrate? I wonder if these combined coatings have ever really been tested. Since all coatings are not formulated the same and they differ from one manufacturer to another, will the adhesion be the same when you combine any of your coatings together?
Today, it is fairly common for finishing shops to buy their finishing materials from more than one supplier. This often happens when a finishing shop is in a rush for certain materials and has to purchase them from a local supplier. These materials may be chemically compatible. However, how does one actually know if they also will have good adhesion to the substrate and to each other? The answer is that you will not really know, unless you do your own simple low-tech test, which is known as the “cross-hatch” test.
|The cross-hatch test is a simple, easy way to check adhesion of coatings.|
What is the Cross-Hatch Test?
The cross-hatch test is very simple and easy to do. Some call it the “stick-and-pull” test, and it has been used by coating manufacturers, finishing and refinishing shops for many years. All you need is a good grade of masking tape and a sharp utility or Exacto knife.
You begin with a 6-inch piece of masking tape that is placed on top of the dried coating, then rub it smooth with your hand. Fold over a small piece of the tape to make a hand tab about 1/2 inch long, which will be used to pull the tape off the sample. (See photos below.)
Next, take your knife and cut through the tape and down through the finish, making two sets of two 1/4-inch lines vertically and horizontally, about 1 inch long. They should look like the Tic-Tac-Toe game pattern. Grab the hand tab on the tape and pull it evenly to remove it from the finish and the substrate. Then inspect the cross hatch to see if any of the finish has pulled away at the cut area, or if it is intact.
When doing this test, you should make complete samples from start to finish, and allow the finished panels to dry or fully cure. Because of the numerous coatings available, the methods of applications and the systems for drying found on the market today, I cannot give a definitive time span as to how long to wait before do your test. I personally prefer doing the test after 7 to 10 days. Then I do another cross-hatch test in two weeks to double-check for any differences. Always keep notes on the samples, so you can make comparisons for future references.
There is another test that can be done sooner, but it is not as accurate. After you pull the tape off, if less than 20 percent of the coating is removed, it is considered “passable.” If more than 20 percent of the coating is removed, it is considered poor adhesion between the coating and the substrate. In that case, you should save the panels and redo the test in 7 to 10 days to see if the adhesion improves. Then do it again in two weeks. You may see a difference that will tell you that you need more time for drying or curing.
It’s not ‘High-Tech,’ but It Works
Although, the cross-hatch test is not “high-tech,” it certainly does work and is recommended by the American Society for Testing and Materials (ASTM) and the American National Standards Institute (ANSI). If you do have a problem, there typically are two main causes of poor adhesion: Either there is something wrong in your finishing procedure or there is a problem with your finishing materials.
To look for the solution, you need to start by reviewing your entire finishing process. Following is a list of possible causes for poor adhesion. Be sure not to overlook anything.
• The wood’s surface was not thoroughly cleaned.
• The wood was sanded too finely.
• Incompatible materials were used in the finishing process.
• There was excess moisture in the wood, finishing materials or your application system.
• Your stain or toners were not thoroughly dry before you began to apply your topcoat.
• Glazes are notorious for causing adhesion problems. Knowing how a glaze is applied and when to coat over the glaze can prevent problems.
• Colorants, additives and other media with too much pigment can cause problems in your coatings.
• Over-catalyzation can cause stress and tension on coatings as they continue to cure out.
• During the finishing process, was the shop’s ambient temperature within the recommended range?
Now, that you know how to do the cross-hatch test, grab a roll of tape, a sharp utility knife and start checking your samples. I hope they all pass.
Mac Simmons is a freelance writer and 50-year veteran of the furniture finishing, refinishing and restoration trades. Mac has written articles for woodworking magazines in the U.S. and in the UK, Austria and Canada. Questions may be directed to him in writing c/o CWB, 400 Knightsbridge Pkwy., Lincolnshire, IL 60069 or via e-mail c/o firstname.lastname@example.org.