The random orbit sander was introduced in the early 1990s and it quickly became very popular. Nowadays, it is the most popular type of sander. The main reason for the success of the random orbit sanders is its versatility, it can be used for most sanding tasks.
The random orbit sander gets its name from the random orbital movement that has been added to the normal spinning of the disc. The random orbits ensure that no swirl marks are created. This also means that the random orbit sander is not sensitive to the direction of the grain.
Several types of random orbit sanders are available but the most popular type is the small palm sander (or palm grip sander). They are compact and lightweight, making it possible to operate them with just one hand. Most palm random orbit sanders use 5-inch discs. Larger models are generally two-hand models, they are larger and heavier but also more powerful.
Two types of sanding discs are used, hook and loop discs and pressure sensitive adhesive (PSA) discs. The hook and loop discs have one big advantage, they can be re-used. If you are going to use multiple grits, you can change the discs and keep on using the old discs until they are completely worn out.
The pressure sensitive adhesive discs are cheaper and they are flatter than hook and loop discs. The hook and loop discs can have trouble making a soft surface completely even. But it is not recommended to try to re-use PSA discs, once you remove the disc, throw it away. The sawdust has most likely ruined the chances of getting the disc to stick to the pad.
Nowadays, almost all popular random orbit sanders use hook and loop discs. They are more convenient to use. You also have some hook and loop sanders that accept a PSA pad so that you can use both types of discs. Note that the hooks on the pad will eventually wear out and the pad loses its gripping ability. You need to replace the H&L pad, it is easy to do but will add additional costs. Generally, the pad can be used for several hundreds of disc changes but only if you follow the recommendations in the manual. For example not using dust collection will quickly wear out the pad.
The discs generally have six or eight holes for the dust collection. A cloth bag is the most popular solution for dust collection. They work fine but a canister is a little bit more efficient. But the best solution is to use a shop vac. Note that random orbit sanders have no standardized interface for shop vacuums, you need to check that the sander can be connected to your shop vac.
Note that it is best to operate a random orbit sander by using light pressure. Pressing too hard is generally counterproductive, it removes stock faster but slows down the rotation of the disc. This easily creates swirl patterns on the surface.
If you are not sure what kind of sander you should buy, a random orbit sander is generally the best choice. It is the most versatile type of sander, it can be used for virtually all sanding tasks. A compact and lightweight 5-inch orbital sander is generally better than a 6-inch model, unless you have a lot of heavy sanding to do.