Disc sanders come in different sizes, from small portable models to large stationary models. They all have a disk with sandpaper attached to it, hence the name of the sander. Disc sanders can be used for many sanding tasks but they can’t be used for fine sanding.
Stationary disc sanders can be divided into two categories, bench mounted disc sanders and floor standing disc sanders. You also have combined disc and belt sanders.
Typically, three different grits are used, coarse, medium and fine. Coarse grit is used to remove stock quickly. Medium grit is used to get a relatively smooth surface which can be finished by hand sanding or using a finishing sander. Fine grit is used to get a smooth surface and to sharpen tools. The surface still requires some hand sanding to get it perfectly smooth and to remove swirl marks.
Portable disc sanders are small, the disc size is typically between 5 and 7 inches. Larger disks are very difficult to control. The portable disc sander is popular in automotive industries and is often powered by compressed air rather than electricity. Portable disc sanders are not common in woodworking but they are very good for tasks such as removing paint from walls. Note that unlike the random orbit sander and the orbital sander, which add an orbital pattern to the rotating movement, the disc sander just spins the disc. This means that you have to be more careful when using a disc sander.
For woodworkers, a bench mounted disc sander is a very useful machine. It does not take up as much space as a floor standing model and is cheaper as well. But you need a solid model, made of steel and cast iron, the cheap models often vibrate a lot.
Large floor standing disc sanders are mainly found in workshops but you have some cheaper models, suitable for home use as well. The most expensive models have a lot of advanced features but they are far too expensive for woodworking hobbyists. Unfortunately, many of the cheap models are flimsy which makes them less funny to use. On the whole, floor standing disc sanders are best for professional workshops.
The combined disc and belt sander is compromise but it saves both money and space. Most of the combined disc and belt sanders are not really professional grade but they are generally good enough for most woodworking tasks at home. Just remember that a combined disc and belt sander is seldom as good as a real disc sander.
Sanding generates a lot of dust, so make sure that you have good dust collection. Unfortunately, far from all disc sanders come with dust collection. Remember the basic rule stationary disc sander rule, always sand on the downward motion side.