Reprinted with permission from Custom Woodworking Business.
"Pencil Hardness" is a simple test that can tell you the hardness of the coatings you are using.
By Mac Simmons
Although some people may consider it to be low tech, using pencils to test hardness is still a method used by many coatings manufacturers and some finishing shops. The test uses special pencils with different degrees of hardness to scratch the coating, which then determines its hardness. If you are not familiar with this test, then you may be surprised at the comparison ratings for many of the coatings commonly used for finishing wood. Looking at the pencil hardness ratings of different coatings is one criteria to help a woodworker decide which one to select for a particular job.
The simplicity of the pencil hardness test is its great advantage. The special pencils used for doing the test are made of various combinations of graphite and clays, baked in ovens to harden their compositions. By adjusting the amounts of these two organic materials, the pencils are either very soft or quite hard. Sets of the pencils are available at most art supply stores or office supply centers.
A complete set will have the following pencil gradings, starting with the hardest — 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H, F, HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B and 9B (the latter is the softest pencil; it contains the most graphite and the least amount of clay). The “H” stands for “hardness” and the “B” stands for “blackness,” which comes from the amount of graphite used in the softer pencils.
There is another pencil grading system that can be used, which is equivalent to the B grades: a #1 pencil is equivalent to B, #2 is the same as HB, #2-1/2 is F, #3 is H, and #4 is a 2H. (The most commonly used writing pencils are the #2 or the HB grade, which are softer and leave darker markings when used for writing or drawing.)
To prepare a sample for testing, the wood should be well sanded, clean and free of all dirt and dust particles. The coating's thickness should be between 1.0 and 1.5 mils (a mil is 0.001 inch). After the sample has been coated, it should be allowed to dry for seven days before the pencil hardness test is done. In some cases, it may be useful to note the times when the testing was done along with the results.
The ambient temperature where the tests are done may have an effect on the hardness of the coatings, as this can be a factor in the drying times. Some coatings may get harder after a week's time and, if retested, would show a higher pencil hardness rating after more time elapses. If you really want to find out a coating's hardness, you can repeat the test at any time later on the original samples. It is a good idea to date and write in the pencil hardness, then save the samples for future reference.
Although the test itself is simple and easy to do, it gives uniform results and is dependable because the pencils are uniform in their grading. For a minor expenditure in time, you can measure your coating's hardness accurately.
To perform the test, just select a pencil and make a line about one inch long on the wood sample. If the pencil leaves a scratch, then take the next softer pencil and do the same thing. The number of the first pencil that you use after you have made a scratch in the coating that doesn't leave a mark is considered the "pencil hardness" of the coating. (There are some coatings that are so hard that even the 9H pencils will not scratch them. All of those coatings get a 9H pencil hardness rating to designate their hardness.)
Shown below is a listing of the pencil hardness of some different common coatings. The information was taken from various manufacturers and some tests I did myself. It can be used as a general comparison.
Knowing the pencil hardness of the coating you select can be helpful information to pass along to customers — if they want a coating with a low pencil hardness, then they should be advised to take extra care of their furniture, because these lower-rated coatings will dent, scuff and scratch faster than a coating with a higher PH rating. (But even if the coating used has a higher rating, care should still be taken on all coatings, as no coating is impervious to damage.)
Using the pencil hardness test can also be useful in comparing products from different manufacturers. For example, a polyurethane from one company may have a pencil hardness of 3H, while polyurethane from another supplier may be different. I suggest you do your own testing to know the true hardness of the coatings brand you use.
Of course, the hardness of any coating is only one of its features. There are times when using a softer coating may be more beneficial to the overall performance of the finish. For example, the ability of a softer coating to flex with normal climactic contractions and expansions of wood, which are caused by weather and changes in temperature conditions, may be more desirable in some applications.
Besides the hardness of a coating, there are other factors that can be tested and are equally important. These include: abrasion resistance, reverse-impact resistance, direct-impact resistance, cross-hatch adhesion, oxidation, gloss retention, yellowing, blistering, cold checking, dry times, chemical and solvent resistance (using both the rubbing and spot time tests), salt-spray resistance, VOCs and HAPs contents, humidity resistance, and acid and caustic resistance.
Most coatings are formulated for specific types of finishes, various conditions or different substrates. So use the pencil hardness test as one criteria for selection. But do not judge any coating by pencil hardness alone, as there are many other significant characteristics to consider.
Pencil Hardness for Common Coatings
TYPE OF COATING: PENCIL HARDNESS
Catalyzed polyester: 9H
Catalyzed polyurethane: 9H
Catalyzed modified acrylic polyurethane: 4H
Catalyzed acrylic polyurethane: 2H
Water-based polyurethane: 3H
Water-based urethane/isocyanate catalyst: 2H
Conversion varnish: 4H
Low-VOC lacquer: 3H
Low-VOC catalyzed lacquer: 2H (24 hours)
Urethane/nitrocellulose lacquer: F (24 hours)
Water reducible lacquer: 2H
Tung oil/polyurethane wipe-on finish: 2H
Water-based polyurethane wipe-on finish: HB-F
Precatalyzed aerosol: 3B
Waterclear acrylic aerosol: 3B
Clear shellac aerosol: 3B
Polyurethane/nitrocellulose aerosol: HB
Nitrocellulose aerosol: 3B
Amber (orange) shellac 1 lb. cut (waxed): 3B
Reprinted with permission from Custom Woodworking Business.