The grit size of sandpaper is prominently displayed on the back of better grades of paper. Don’t use an ungraded paper.
In any discussion of sandpaper the words “coarse,” “medium” and “fine” always come up. Open any book on finishing or refinishing, or any do-it-yourself home reference book and you will find these terms used as a guide for the grade of paper you should use for a given project. But in the very specific world of professional woodworking and finishing, these terms have absolutely no meaning.
Read Fred’s Rough Stuff No. 1 – An Introduction to Sandpaper.
You don’t generally buy good shoes in “large,” “medium” and “small,” and you shouldn’t buy sandpaper that way, either. Coarse, medium and fine are relative terms and don’t tell you enough in absolute terms. What is coarse in one application would be deemed fine in another.
Sandpaper is graded in absolute terms by the number of holes per square inch in the mesh that was used to screen the particles that are glued to the backing. The lower the number of holes per square inch the larger the particles that can pass through. Therefore, a paper graded as “40 grit” used a mesh of only 40 holes per inch while one graded as “320 grit” had eight times that many and the particles are only 1/8 as big, making it a much finer abrasive than a 40 graded paper.
In common usage, paper is normally available in 36, 40, 60, 80, 100, 120, 150, 180, 220, 280, 320 and 400 grits. The lowest five grits—36 through 100—are usually used to radically adjust the size and shape of a piece of wood, such as rounding off edges and quickly (and roughly) decreasing the thickness. These grits leave deep, visible marks in the same category as saw marks and planer chatter that must be eradicated before finishing can proceed. Successively finer grits remove the traces of the rougher grits previously used, but a practical limit is soon reached. 120 should be last grit used on bare wood to remove earlier dimensioning marks.
So what are the other grits used for? Primarily, for adjusting intermediate steps during the finish process and very seldom are they used on bare wood. In fact most wood shops only use 80, 120, 220, 320 and 400, a narrow selection that nevertheless covers the entire range of usage.
220 is most often used to sand the excess of paste wood filler when filling the pores of walnut and mahogany to get that smooth effect on open grain woods. (If you are unfamiliar with the process, I will be happy to e-mail you columns on the subject.) Sometimes 220 can be used to sand a shellac wash coat or a first coat of sanding sealer, but it is a little coarse for that use and can easily burn the edges of delicate pieces. A more appropriate paper for sealer and first coats of lacquer, shellac or urethane is 320—a much finer paper that will not leave deep scratches in the early finish coats. Sanding between intermediate coats of lacquer, shellac, urethane or varnish can also be accomplished with 320. For sanding intermediate coats higher up in the finish schedule, you can use 400, which is also ideal for sanding out the final coat after it has cured to remove minor dust and contamination from the finished product. Sanding with 400 should then be followed by a brisk rub down with 0000 steel wool, followed by buffing to taste and waxing.
But there is one more consideration in selecting a suitable sandpaper for your project; that is choosing the right type of abrasive, as well as the right grit and weight.
The reddish paper on the left is production grade aluminum oxide paper for use in carpentry work. The almost white paper on the right is silicon carbide, the best paper for furniture work. The black paper in the center is for wet sanding final finishes.
The cheapest sandpaper is usually known as “flint” paper, since it uses a yellowish type of natural sand similar to flint. While inexpensive to purchase, it is expensive to use because it wears out so quickly. This lack of staying power means it is almost never the choice for any project that requires more than a few square inches of sanding. Another natural abrasive paper is known as “garnet” paper, which reddish in color and more expensive and more durable than flint but still not up to professional standards.
The most common sandpaper sold in home improvement stores is aluminum oxide paper. This paper, recognized by its brownish-red tint, is more expensive yet, but is getting close to being the real thing. This paper is often called “production paper” and is excellent for short term, smaller, finer applications.
The real pro in the line of sandpapers is known as silicon carbide. This is a synthetic abrasive and is dark gray (or black in the waterproof version) and very light gray (almost white) in the “dry” variety. This is the choice of most woodworking professionals in spite of the initial cost because it is the most long-lasting, durable and, therefore, in the long run, the most cost-effective. Since the abrasive is a manufactured product, rather than a natural mineral, the particles on the silicon carbide paper are more uniform in size and shape than those used in other types of paper and deliver the most consistent performance, from sheet to sheet and from pack to pack. The major manufacturers of professional grade silicon carbide paper are 3M and Klingspor.
White silicon carbide paper in the appropriate grade is the recommended paper for virtually all wood projects. But it can be a little difficult to find. Not many home stores carry it because of its cost—in many cases in excess of $1 per sheet—but good paint stores and higher-end woodcraft/hobby type stores make it available, as do some of the better mail-order supply houses. It’s worth the hunt.
Fred Taylor is a antique furniture Worthologist who specializes in American furniture from the Late Classicism period (1830-1850).
Send your comments, questions and pictures to me at PO Box 215, Crystal River, FL 34423 or firstname.lastname@example.org.
Visit Fred’s website at www.furnituredetective.com. His book “How To Be A Furniture Detective” is now available for $18.95 plus $3 shipping. Send check or money order for $21.95 to Fred Taylor, PO Box 215, Crystal River, FL 34423.
Fred and Gail Taylor’s DVD, “Identification of Older & Antique Furniture,” ($17 + $3 S&H) and a bound compilation of the first 60 columns of “Common Sense Antiques,” by Fred Taylor ($25 + $3 S&H) are also available at the same address. For more information call 800-387-6377, fax 352-563-2916, or e-mail email@example.com.
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