Grit or grade is the size and amount of aluminum oxide, silicon carbide, garnet or other abrasive material glued to sandpaper. The lower the grit, the rougher the sandpaper. The higher the grit, the smoother the sandpaper. Generic sanding instructions typically direct people to begin sanding with low-grit sandpaper, and work your way up to high-grit. But conventional wisdom doesn't fit every application.
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Lower-grit sandpapers -- 60 to 80 -- can do more harm than good, creating scratches that weren't there before. Low-grit papers are aggressive. Use low-grit sandpaper, 50 to 80, on rough lumber, to remove deep gouges and pits or to remove a previous finish. Keep them moving to avoid sanding depressions and deep dips. Never use them on hardwood plywood. Low-grit sandpaper can damage the veneer or remove it completely.
High-grit sandpapers -- 180 to 220 -- can close wood pores, polish wood smooth and cause stain to blotch or fail to penetrate. If your stain is lighter on corners and edges, you've polished them. This is particularly true when sanding harder woods such as maple or hickory. It's rarely to your advantage to use sandpaper finer than 180-grit on any type of bare wood. Top-coat finishes, lacquer and varnish, create their own finish. The appearance of the finished product has little to do with how fine you've sanded the bare wood. If it's smooth and scratch-free after using 100- or 120-grit sandpaper, it's ready to finish.
To ensure consistent finishes, woodworking and cabinet shops often use nothing but 100- or 120-grit sandpaper. Oak, cherry, walnut, hickory and the majority of lumber and plywood products have typically already been sanded or planed smooth, and it doesn't need anything but 100-grit. This includes weather-resistant wood such as cedar.
For cabinetry, moldings, trim or other wooden furniture, a final sanding with 100- or 120-grit sandpaper by hand, parallel with the grain, is sufficient for a consistent finish. If you want to spend extra time sanding with 180- or 220-grit, it typically won't hurt anything, but it's time-consuming and unnecessary. If your project has stain blotches, it's an indication that you might have sanded it too much with light-grit sandpaper.
Planers, surfacers, joiners, drum sanders and woodworking equipment can leave shallow dips, pits, gouges or feathering on lumber or plywood. Using 100-grit sandpaper on an orbital sander is all that's necessary to smooth it. Lower grits, such as 60 or 80, are too aggressive, remove too much material and create scratches. After using an orbital sander with any grit sandpaper, always follow up with a hand block and 100-grit sandpaper, specifically to remove delicate swirls caused by the rotating head.
When to Use High-Grit Sandpaper
Use fine-grit papers, 180 to 400, to lightly sand between coats of lacquer or varnish for the smoothest finishes. If you're finishing tabletops, a final pass with 180- or 220-grit sandpaper on bare wood can help you achieve a glassy surface. You're not sanding it any deeper but just removing small wood fibers from the surface. Fibers aren't really a problem on cabinets, chairs or furniture parts, but when allowed to settle into a glassy topcoat that resembles water, they can become visible. Take it one more step and gently wipe the surface of a tabletop with a sticky tack cloth after sanding to ensure the fibers are removed.
For most woodworking projects, the use of belt sanders is best left to the professionals. Belt sanders are aggressive, remove material fast and can create dips, scratches and depressions. Hardwood floors typically require the use of a belt sander, however. For newer hardwood floors, or floors with little damage or that have not been previously finished, 100-grit sandpaper belts are typically sufficient to smooth and prepare it for finishing. If you're removing a previous finish, or the floor has obvious dents, scratches or wear patterns, the initial sanding can be done with 50-, 60- or 80-grit, ending with 100-grit.