There will never be a project in your shop that doesn't require sanding prior to applying a finish. But without an understanding of the process, or with poor technique, your efforts at creating that perfectly smooth surface may prove frustrating. So let's review some basics.
Start rough, finish smooth
The rough surface on a board's face caused by mill marks, shown below, scratches, or an uneven grain can be flattened by a plane, scraper, or sandpaper. But the plane and scraper require regular sharpening to produce a consistent surface smoothness. Fresh-out-of-the-box sandpaper, however, uses standard grits that make it easy for any woodworker to follow an orderly process for success.
Choose the true grit
Because sandpaper scratches a surface to smooth it, shown below, it's important to begin with the right grit based on the condition of the wood's surface and whether a stain or a topcoat of finish will be applied. Sandpaper can commonly be found in 80, 100, 120, 150, 180, and 220 grit. Grits below 80 are generally used in paint removal, and those above 220 are for sanding between coats of finish.
The coarse grits of 80–100 make quick work of removing mill marks and deep scratches, but still leave behind visible marks. Sanding with 150-grit paper provides a good surface for staining since the very faint scratches create places for pigment in stains to lodge.
When applying a finish without staining, work up to 220-grit paper. The scratch pattern is not visible to the naked eye, yet still provides the "grip" a top coat needs for good adhesion.
I've found it more efficient to not skip any grits. As an example, if I plan to a stain a board with pronounced mill marks or surface imperfections, I begin with 80 grit and progress through 100, 120, and finally 150. Each successive grit gradually removes the previous grit's scratches.
Turn on the power
Random-orbit sanders make sanding a lot easier, and with the right technique, more consistent. To use one most effectively, slowly move the sander in the direction of the grain without pausing to remove specific spots, which can cause dips in the surface. Make sure you sand evenly, shown below.
Repeated sweeps across the entire surface at a rate of 6–12" per second will evenly remove surface flaws. Use a sander with dust collection, preferably hooked to a vacuum, shown below.
Random-orbit sanders operate most efficiently when only the weight of the sander bears on the workpiece. Applying excessive pressure not only slows the rotation of the pad (and thus, the sanding process) but can also cause swirl marks or elliptical thumbnail-scratch patterns.
Because these sanders can't get into corners and may leave swirls, hand-sand the final grit, shown below, moving the abrasive in the direction of the grain. This process doesn't take long because the power sander has done most of the hard work. But it gives the entire project a consistent sanded surface, leading to an even stain or finish.
From aluminum oxide to silicon carbide, there are a number of distinct types of sandpaper. Learn more here: woodmagazine.com/abrasivetypes