Sandpaper is useful for any number of different tasks, including preparing metal or wood to accept paint, abrading finishes off surfaces, removing rust from metal, and smoothing wood. Later, we'll give specific information for selecting sandpaper, but first, a little background.
When you go shopping for sandpaper, you'll find sheets designed for sanding by hand—they're generally, but not always, 9 x 11 inches—as well as products for use with power tools, in the form of belts or Velcro-backed discs, or paper shapes cut to the size of the tools. Packages of sandpaper sheets may contain several pieces all the same grade, or an assortment of grits. The packaging will be printed with the manufacturer's recommended use—from general purpose to woodworking to wet-or-dry—to help you select the best sandpaper for your task.
Grit size refers to the size of the particles affixed to the backing material. All sandpaper packaging is labeled with a grit number; this figure is also usually printed on the back of the sandpaper itself. Grits run from 24 to 2,000 and beyond. (Some manufacturers also label their packaging with words like "extra-coarse," "coarse," "fine," etc.) The lower the number, the larger the aggregate and the rougher the sandpaper. Rough sandpaper takes off more material faster but leaves deeper scratches in the material.
The idea, depending on your task, is to start by sanding the surface with a coarse grit to remove material fast, then keep working your way up to finer and finer grits to progressively remove the scratches left by the previous grade of sandpaper until you achieve a smooth surface. However, your project may require only one grade of sandpaper because the scratches it leaves behind may not be visible—for example, if you're applying paint over the sanded surface.
Most household jobs can be accomplished with sandpaper in 60- to 220-grit.
Types of Abrasives
Despite its name, sandpaper isn't made of sand. Instead, manufacturers produce their products with both synthetic and natural abrasive grains. As you sand, these grains fracture, creating new edges that remove material. Different abrasives have different properties.
- Garnet is a natural material. It works especially well for hand-sanding wood, but this type of sandpaper wears out fairly quickly. This sandpaper is commonly reddish-tan in color—although manufacturers have been known to dye their sandpapers some outlandish colors!
- Emery is perfect for sanding metal, but it is too soft for other uses.
- Silicon carbide is the most durable man-made material, and it works well for all applications, including all kinds of wood, fiberglass, and plastic. It's especially good for metal. You can spot sandpaper made from silicone carbide at a glance, because it's usually dark gray or black.
- Aluminum oxide sandpaper is generally pale beige or brown in color. This durable synthetic material works well for all applications, including metal, plastic, drywall, bare wood, and sanding off paint.
- Zirconia alumina is a manmade ceramic abrasive used primarily for power sanding applications.
Other types of abrasives exist, but they have more specialized applications so you really don't need them for around the house.
Types of Backings
Sandpaper was originally backed with paper—hence the name—but these days you'll also find it backed with fabric or film, which helps make it longer-lasting. Fabric-backed sandpaper can be used dry or wet. Sanding with water or some other lubricant helps to carry away the dust formed during sanding, reducing scratches, and keeping the dust out of the air. It also prevents the sandpaper from getting clogged so it lasts longer. Do not use water when sanding wood, however.
Cutting sandpaper: If you’re using a sanding block, you’ll want to cut the paper down to size. Paper-backed sheets can be torn by hand. Just fold the sheet into quarters, score the lines, and tear. Fabric-backed sandpaper can be cut with scissors, but it will dull the blades terribly.
Open-Coat vs. Closed-Coat Sandpapers
The terms open-coat a_nd _closed coat define the density of the abrasive on the backing. Open-coat means that only 40 to 70 percent of the surface is coated with abrasive. The space between the abrasive grains gives the dust someplace to go, so that the sandpaper won't clog as quickly as closed-coat sandpaper. The surface of closed-coated sandpaper is completely coated with abrasive, so dust clogs it quickly. Closed-coat sandpaper is more suitable for hand-sanding or with hardwoods and metal, while open-coat works well for power tools and with soft woods such as pine.
Always wear protection. Sanding produces dust, which isn't safe to inhale, so always wear a dust mask, respirator, or some other breathing protection, and put on safety goggles to keep it out of your eyes.
Selecting the Right Sandpaper for the Job
Different tasks call for different sandpapers:
- Shaping wood: Garnet, aluminum oxide, or zirconia alumina, 80-grit or lower
- Removing mill marks and other flaws in wood: Garnet, 100-grit or lower
- Sanding hardwoods: Aluminum oxide, 100- through 220-grit
- Smoothing bare wood before painting: Garnet or aluminum oxide, 100- through 220-grit
- Smoothing bare wood before staining: Garnet or aluminum oxide, 100- through 150-grit
- Sanding painted or primed wood in good condition, before painting: aluminum oxide or silicon carbide, 180-grit
- Sanding painted or primed wood in poor condition, before painting: aluminum oxide or silicon carbide, 80-grit; follow that with a coat of primer
- Removing raised wood fibers: Garnet, 180- through 320-grit
- Sanding the edges of a door that sticks: 80-grit
- Sanding old floors: Garnet, 80-grit or lower
- Grinding burrs off of metal: Zirconia alumina, 80-grit or lower
- Removing rust from metal: Aluminum oxide, 100-grit or lower
- Preparing bare metal in order to paint it: Aluminum oxide, 150-grit
- Stripping paint or varnish: Any type, 100-grit or lower
- Smoothing between finishes (such as lacquer, shellac, or paint): Aluminum oxide wet-or-dry, 220- or 280-grit
- Sanding drywall: Any type, 80- to 150-grit
- Smoothing joint compound: Start with a 120- or 150 -screen, then sand with silicon carbide 150-grit
- Sanding grout to remove stains: Any type, 100-grit