You may have some nice trim or a cabinet or furniture piece you need to sand in preparation for painting or staining. If you choose a sandpaper grit that's too coarse, you can end up scratching the surface or muddling the nice, crisp details of the woodwork. If you choose a grit that is too fine, you may need to sand for days, or you can over-sand and affect how the wood accepts stain. Welcome to the world of sandpaper grit.
Grit Numbers Go Backwards
Sandpaper grit is sized by a gauge number, and the lower the number, the larger the grit. The larger the grit, the coarser, or rougher, the sandpaper. So, the best way to look at this is simply to remember that the numbering runs backward.
The grit number of sandpaper is almost always clearly printed on the back of the sandpaper itself. Grit numbers range from 24 all the way up to 1,000, although most homeowners and DIYers will never use grits at the top or bottom of the scale.
You might think that the gauge (grit number) relates to the number of grains deposited on a square inch of sandpaper, but it actually refers to the number of holes per square inch in the screen that is used when sieving the abrasive grains during the sandpaper's manufacture.
Why Grit Matters
Coarser, or lower-grit, sandpaper removes wood and other materials faster and with less effort than finer sandpaper.
It does this by scratching away the fibers on the material's surface. Aggressive scratching is good when you want to remove a lot of material, shape an edge, or remove old paint or blemishes, but the coarse sandpaper leaves deep scratches in its wake. If you're not careful, this can easily damage your project, as the scratches can be difficult to remove without taking a lot of the surface with them.
At the other end of the spectrum, very fine sandpaper removes a tiny amount of material, with the effect of smoothing the surface. And the finer the paper, the smoother the surface. The concern here is that if you switch to a fine paper too soon, you'll spend a lot of time sanding and not get very far. Also, sanding wood with too-fine paper, or sanding too much, can actually burnish the surface, creating overworked areas that can hinder absorption of stain and other finishes, with splotchy results.
The trick, then, is to start with the highest grit that will meet your needs relatively quickly, then move up to progressively higher grits as the surface gets closer to the finished product—and stop when it's smooth enough for your liking; don't overdo it.
Normal Grit Ranges
While you can find scores of differently graded sandpaper available, most sanding projects call for papers in the following grit ranges:
- #60–#80: Coarse—Cuts through old paint and rough edges with relative ease. Also shapes and rounds edges. Not recommended for fine details or edges and corners you want to keep sharp. Also, be very careful using this on plywood, which has thin face layers that are easy to sand through.
- #100–#150: Medium—The most often used gauges of sandpaper. It is hard to go wrong with sandpaper grits in this range. You can work down difficult materials by applying more pressure to your workpiece. Or, you can preserve fine materials by letting up on the pressure. Generally used for bare wood surfaces. A final sanding with 150-grit paper is commonly recommended for wood surfaces that will be painted; it leaves a little "tooth" to the wood surface for the paint to grip onto, and usually sanding more doesn't yield a smoother painted finish (this doesn't apply to high-gloss finishes that require sanding the paint itself).
- #180–#220: Fine—You seldom use these grades of sandpaper on the first run-through, unless the surface is already smooth the touch. Grits in this range are typically for second or third sandings. Sometimes, fine grit sandpaper is used to "roughen" down glossy paint in preparation for applying another coat of paint. Bare wood that will be stained usually should not be sanded with higher than 220-grit paper.
- #320 and Up: Ultra-fine—Use these for another level of smoothness on all types of materials; with wood, ultra-fine grits usually are reserved for smoothing painted surfaces between coats. Many finer grits are used for wet sanding, which creates a fine, gritty slurry that complements the sandpaper's efforts at smoothing.