Testing wood finishes

With a few simple tests you can check if a finish has the qualities you want
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Some finish manufacturers provide lots of information about their products that make it easier for you to choose among brands or within a brand to get the qualities you want. But many don’t. Companies like BYK-Gardner sell very sophisticated measuring instruments that allow you to make accurate measurements yourself. But most of these instruments are expensive.

So here are some inexpensive and easy tests you can perform in your shop to determine key comparative differences. In most cases you’ll want to do the tests on scrap wood. For the most accurate results let the finish dry for at least a couple of weeks in a warm room before performing the tests.

Scratch resistance

The ability of a finish to resist damage from coarse or sharp objects is one of the most important qualities you’re likely concerned about.

To a large extent you can know the comparative scratch resistance simply from the category of finish. For example, oil-based polyurethane varnish and catalyzed lacquers are much more scratch resistant than shellac and nitrocellulose lacquer, and usually a little more scratch resistant than acrylic and polyurethane water-based finishes.

But what about differences among brands within each finish type? Or what about the comparative difference between a water-based polyurethane and an oil-based polyurethane – that is, how much scratch resistance are you actually giving up using a water-based polyurethane?

To determine a finish’s scratch resistance, purchase a set of architect’s drawing pencils ranging in hardness from about 2B (soft) to 5H (hard). Sharpen each pencil using a knife so you leave the sharp cylindrical edge of the lead intact. If you damage this edge, or if it becomes worn, sand it flat, holding the pencil 90 degrees to the sandpaper.

Beginning with one of the softer pencils, hold it as you would for writing and push it forward an inch or two across the finish. Maintaining equal pressure, follow with pencils of increasing hardness until you find one that cuts into the finish. The hardness rating for that finish is the rating of the previous pencil – the hardest lead that doesn’t cut.

Water resistance

Another quality you may want is water resistance. All finishes are more water resistant the thicker they are applied. So testing won’t be very revealing except on thin finishes such as oil or wax, or any film-building finish applied with just one or two coats.

To test for water resistance, make up a sample board with the same number of coats applied in the same manner as you intend to use on your project. Then place a small puddle of water on the surface and cover it with a metal or glass cup or jar to prevent evaporation. Check every 10 minutes or so until you notice cracks or discoloration in the finish. Rate the finish at the most recent previous time before the damage occurred.

The most vulnerable surfaces to water damage are the top and bottom edges of cabinet doors and the raised panels within the door frames because these surfaces often receive a reduced finish build. They are especially vulnerable just below a sink. To test these surfaces, stand a finished sample door on a sponge lying in a pan of water. Check the finish around the edge every so often until cracks appear, the finish delaminates, or there is some discoloration.

Heat resistance

Resistance to damage from hot objects is an important quality for tabletop and counter surfaces in kitchen and dining areas. Again, finish types tell you something. For example, oil-based polyurethane and catalyzed finishes should pass almost any test. But shellac, lacquer and water-based finishes will likely be vulnerable.

There are two tests for heat resistance.

To test for dry-heat resistance, place a metal cup or pan containing water heated to just below boiling on a dry finish. Check under the container as it is cooling for discoloration, splits or indentations.

To test for wet-heat resistance, do the same as above but this time place a cotton or cheesecloth wetted with the same hot water under the cup or pan.

Chemical resistance

Eating surfaces are also vulnerable to staining from a number of household liquids independent of the thickness of the finish. In other words, the finish itself may become stained. Common products that can cause a problem include mustard, vinegar, orange juice, lemon juice, grape juice, ketchup, coffee, tea, wine and 100-proof alcohol. (You can use denatured alcohol mixed half with water instead of liquor for the alcohol test.)

To test a finish for resistance to each of these liquids place a number of drops on the finish and sponge them off one at a time at short intervals (generally several minutes apart) until the finish under the drops becomes dull, discolored, shows cracks, or the wood underneath becomes stained.

The resistance of the finish to damage is rated at the last time before the damage appeared.

Adhesion

How well a finish sticks is not usually an issue when the finish is applied directly to sanded wood as long as the sanding isn’t too fine. But the bond can be an issue over stains, sanding sealers, glazes and pore fillers.

To test for adhesion, cut through the dried finish with a series of horizontal and vertical lines, making a crosshatch pattern, about 3/4” long and 1/16” apart using a razor blade. Then place a strip of masking tape over the scorings and press it down with your finger.

Pull the tape up quickly and observe how clean the cut lines remain. The more jagged the edges of the cuts, the poorer the finish is bonded. If your finish has adhered well, the edges of the cuts should still be fairly clean.

If you are testing to determine if you have weakened the bond with the stain, sanding sealer, glaze or pore filler you have applied, test the bond over these compared with the bond of the finish applied directly to the wood. Ideally, the edges of the cuts should look the same.

Bob Flexner

Bob Flexner

Bob Flexner is author of “Understanding Wood Finishing” and “Flexner on Finishing.”

This article originally appeared in the March 2018 issue.

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