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What you really need to know about sandpaper.

Transforming a coarse, grainy wood surface into one as smooth as a baby's skin requires time, patience, and that basic but indispensable woodworking tool--sandpaper.

Although it may seem boring and time-consuming, sanding is deceptively important work, and crucial to a well-finished project. One woodworking expert estimates that half the cost of a grand piano is in its shiny finish, obtained by sanding the wood (and applying layers of lacquer).

You may have many different reasons for sanding--from preparing a door for a new coat of paint, to readying a piece of unfinished furniture for staining, to restoring an antique. No matter what you sand--and whether you sand by hand or with power equipment--the best finish results if you sand in an orderly progression.

SANDING UP CLOSE

On a microscopic level, sanding is like smoothing and sharpening wood by scratching its surface over and over with progressively smaller cutting tools. Most woodworking projects require at least three sandings. You start with coarse sandpaper, progress through the medium grades, then finish with sandpaper that feels almost as smooth as typing paper.

The coarse paper does the general job of leveling the surface but leaves wide, deep grooves. (If you look at the sanded wood under bright, low-angled light, you'll see deep parallel rows, much like furrows in a plowed field.) A progression of finer sandpapers removes these furrow tops until the wood surface is smooth and level.

That's where having patience comes in. If you use too fine a paper too soon, it rounds the furrow tops without reaching the troughs created by the coarse paper.

In all cases, it's important to remove the sawdust between sandings. Top woodworkers vacuum all wood surfaces whenever they switch sandpaper grades, then follow with a pass from a tack cloth or a rag impregnated with mineral spirits. The mineral spirits evaporates quickly and doesn't raise the wood grain as water does.

But cleaning doesn't apply just to the wood surfaces. You'll extend the sandpaper's life by cleaning it with special rubber cement blocks, especially if you're using a power belt or disk sander. These blocks of dried rubber cement remove wood particles that clog the sandpaper and inhibit it from sanding properly. (There's often perfectly good sandpaper beneath the wood particles.) The blocks cost about $10 at good woodworking stores and from some mail-order sources.

You can oversand wood on occasion. Sanding some hardwoods (such as cherry) with extra-fine paper virtually closes the wood's pores: it won't accept a stain. The rule of thumb is the finer the grain, the lighter the stain. There are times, therefore, when you should stop sanding before you get to fine sandpaper, and darken the wood with some stain.

TRUE GRIT--OR HOW TO MEASURE ABRASIVES

When you run your hand across different grades of sandpaper, some feel bumpy while others are relatively smooth. What you feel reflects the coarseness of the sandpapers' abrasive particles--or grit. In sandpaper lingo, grit refers to the smallest openings through which the abrasive particles might pass. For instance, "220 grit" means that the abrasive particles will pass through a screen with 220 openings per linear inch.

Sandpapers range from about 60 to 1200 grit. The lower the number, the coarser the sandpaper. Although you might find sandpapers below 60 grit, they aren't recommended for woodworking. And it's unlikely that you'll need sandpapers above 600 grit.

You normally use coarse papers (60, 80, and 100) to level the wood at the beginning of the sanding process, then go to medium papers (120 to 180). This grade may prove sufficient for finishing soft woods (such as pine or redwood) that won't benefit from finer sanding. But you need to complete most sanding with fine papers (220 to 280). Extra-fine papers (320 to 600) give some hardwoods a glassy finish and are good to use between coats of paint or sealers.

Have you ever noticed the terms open coat or closed coat on the back of sandpaper? They refer to the density of the abrasive material on the sandpaper surface. On open-coat papers, only about 70 percent of the surface is covered with abrasive, leaving space to accommodate sawdust build-up. Closed coat means that the entire surface is covered, sometimes leading to faster clogging of the paper with sawdust.

SANDPAPER TYPES

A glance at a rack of sandpapers in any hardware store reveals sandpapers of many different colors and textures, with both cloth and paper backings. Oddly, not any are faced with sand.

In earlier times, hide glue was used to bind real sand to bark or paper. But modern sandpapers are of manmade abrasives as well as natural ones such as flint and granite. For most needs, there are four general kinds to choose from. They vary in price, durability, and usefulness.

Flint. The original form of mass-produced sandpaper, flint paper, is no longer popular. Flint paper is coarse, and best for rough work such as removing old paint or finish. (These tasks clog finer-grit sandpapers.) The particles of flint (a form of quartz) dislodge easily, and unless the wood surface is cleaned they can mar the wood upon subsequent sanding.

Garnet. Another natural mineral, garnet is harder and sharper than flint and recognizable by its red color. The paper-backed sheets are relatively inexpensive and come in a wide range of abrasiveness: coarse and medium grit are recommended. Garnet papers tend to have a shorter life than manmade types. They're good on all woods.

Aluminum oxide. This synthetic product is probably the most popular abrasive for woodworking (and for use on most metals). It's made from bauxite and brown to brownish red in color. It's longer lasting, sharper, and more uniform in grit size than garnet paper. It comes in a wide grit range--coarse, medium, and fine categories. Some finer grades are available with a coating of zinc stearate that reduces sandpaper clogging. This type of paper is good for finishing hardwoods like cherry or maple.

Silicon carbide. This is another manmade abrasive, but it's faster cutting and more expensive than aluminum oxide. It resharpens itself as it wears, and is well suited for sanding glue-impregnated products such as particleboard and plywood. It's used most often to sand between coats on painted or finished surfaces. Generally waterproof, it's good for wet sanding. It also is available with zinc stearate coating.

SANDING RULES

Never sand across the grain. This cardinal sin of woodworking is the hardest to atone for. Of course, this is difficult to avoid in projects with joints. Many professional woodworkers, therefore, try to sand all the wood before assembly. (Some even prefer to stain before assembly, which helps avoid glue blocking the stain along joint lines.)

Sanding before assembly also allows you to work on pieces while they are on flat surfaces and easier to handle. And since they haven't been connected, areas that will later become . inside corners are easy to sand. This is especially useful if you're working with a power sander. One hint for after-assembly sanding: use masking tape at joints where different grain directions meet.

Conversely, always sand with the grain. Try to stand at an end of the work piece and sand forward and back with an even pressure throughout the range of motion. If you stand to the side of a piece of wood, it's difficult to resist the tendency to move your arm in an arc.

Use a sanding block. This tool is essential for all flat work being finished by hand, because you cannot rely on the shape of your hand alone to act as a flat, even backing for sandpaper. You can make a sanding block by wrapping sandpaper around a flat piece of 2-by-4 that fits comfortably in your hand. Or you can purchase a ready-made holder that has handles for easy grasping. There are numerous styles to choose from.

Sanding irregular surfaces takes more ingenuity, and occasionally, specially shaped tools. Wads of used sandpaper can be used to sand some intricate edges. Strips from used sander belts or emery cloth help smooth turned pieces like table legs, and you can buy lengths of abrasive cord or tape for reaching into tight grooves.

If you feel heat, you're pressing too hard. Let the sandpaper cut the wood with minimal pressure. It may be better to drop back to a coarser paper to remove the wood material than to bear down harder on a finer one. This applies to working with power tools as well. Let the machine do the work, especially with belt, oscillating, or orbital sanders.

Sand the same throughout. This rule adds to the toil, but if you are planning to stain the wood, it's important to sand all pieces with similar-grit sandpapers. With difficult pieces, it's tempting to stop at a coarser grit than you may have used on an easier piece. But if you do that, the stain will soak in at a different rate. A table, for instance, might have dark legs (which are harder to sand) and a lighter top (which is easier to sand).

Don't rely on stain and sealers to finish a project. Rather than hiding an incomplete sanding job, they tend to amplify it. Use your fingers to feel for scratches. Follow up the touch test with a bright, low-angled backlight to check for deeper scratches. Also, if you have completed sanding with a fine sandpaper, let the dust settle, vacuum the room, and apply the stain or finish as soon as possible. A sanded piece shouldn't stand for more than 24 hours because exposed surfaces can pick up moisture from the air.
COPYRIGHT 1992 Sunset Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Whiteley, Peter O.
Publication:Sunset
Date:Oct 1, 1992
Words:1598
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