Woodworker's Journal 2011-35-Winter, страница 20

Starting with a coarse grit (80 or 100), work successively up through finer grit sandpaper. Use a pattern of sanding in alternating angles across the grain, until you finish with 180-grit (the author prefers garnet paper for the final pass) or finer, working with the grain.

Types of Grits

Our example is garnet paper, and it says so clearly. However, it is not always so obvious. Some companies use code words for certain types of grit. Adalox, Aloxite, Imperial, Metalite, Production, and Three-M-ite are all words used for aluminum oxide. Silicon carbide grit may be called Tri-M-ite, Durite, Fastcut, or Powercut. I use aluminum oxide grit for sanding raw wood. It's a sharp mineral that cuts fast and stays sharp. Silicon carbide will work well too, but it is usually more expensive and there is no real advantage to it. But for my final sanding, with the grain, I switch to garnet. Used garnet paper is also handy for "burnishing" end grain so that it absorbs less stain — a common problem when staining both flat and end grain.

The Paper Itself

Next comes the paper's weight. Paper backing goes from A, the thinnest and most flexible, through C, D, E, and F, the thickest. I prefer thin paper backings. They don't "crack" when you fold them and easily conform to curves. When I do want a flat sanding surface, I back them up with a sanding block. Of course, not all sandpaper is made with paper. Other common backings include fiber (disks), cloth (belts), waterproof paper (wet-or-dry sheets) and polyester film, which is also waterproof (disks, belts and sheets).

Closed coat sandpaper means that grit covers 100 percent of the surface of the sandpaper. Open coat means that only 40 percent to 70 percent of the surface is covered. In other words, there are spaces between the pieces of mineral grit. Since each sharp piece of grit acts like a small cutting tooth, the spaces between act like the gullets in a saw blade, clearing out sanding dust (called "swarf') as you work so that the paper does not clog. For woodworking, we use only open coat papers.

Grit Size

Also printed on the paper is the number indicating the size of grit and, accordingly, how coarse the paper is. That seems simple enough, but it turns out there are different grading systems. In the CAMI or ANSI system, the most common, low numbers mean coarser paper, and higher numbers mean finer paper. The FEPA system uses the same numbers, but with a P in front of

them. The grits are identical from 220 and coarser, but increase at a different rate for finer paper. As a result, CAMI graded 600 paper is about equivalent to FEPA graded P1200 paper, but 180-grit is equal to P180. Next is the micron system, whose numbers get larger as the paper gets coarser — just the opposite of the other two systems. Micron papers have the Greek letter "mu" after the number. And, of course, there is the old standby, the naming system that uses words like extra coarse, medium and ultra fine.

Sponges and Blocks

Sandpaper comes in square sheets, stick-on disks backed with pressure sensitive adhesive (PSA) or hook-and-loop fasteners, belts, and even blocks and sponges. The latter are fairly recent additions to the pantheon of sanding choices. Sponges coated with abrasive grit are waterproof and, unlike sandpaper, can be rinsed out and reused. They easily conform to whatever shape you are sanding without tearing or wrinkling. The only down side is that they are substantially more expensive.

Sanding is not fun, but if done correctly, it will improve your finishing results dramatically. And as much as any other aspect of woodworking, a good finish reflects well on the builder. &

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20 sanding: the real key to a great finish