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Discover Mid-America — May 2005

Rough stuff #2: Choosing the right one

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.

You don't buy 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 8 times that many and the particles are only 1/8 as big, 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. One-twenty should be last grit used 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.
Two-twenty 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 send $2 for copies of "Filler Up - Parts I and II.") 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 is 320, a much finer paper that will not leave deep scratches in the early finish coats. Three-twenty can also be used for sanding between intermediate coats of lacquer, shellac, urethane or varnish. Four hundred can also be used for sanding intermediate coats higher up in the finish schedule and is 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 of 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 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. It’s 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's new book "HOW TO BE A FURNITURE DETECTIVE" is now available for $18.95 plus $2 for S & H. Send check or money order for $20.95 to Fred Taylor, PO Box 215, Crystal River, FL 34423.

Fred and Gail Taylor's video, “IDENTIFICATION OF OLDER & ANTIQUE FURNITURE,” ($29.95 includes S & H) is also available at the same address. For more information call (800) 387-6377, fax (352) 563-2916, or e-mail fmtaylor@aol.com.

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