To avoid getting in to a lot of unnecessary detail surrounding sandpaper, we will only talk sandpaper as it pertains to woodworking.
Sandpaper is described in grits. Grits refer to the size of the abrasive particles found on the sandpaper. The higher the number, the higher the number of smaller particles found on the paper. These smaller particles are finer and do not remove as much material. A lower number grit has less particles on the paper which are larger. Low grit paper removes more material.
There is a wide range of grit numbers available, from 24 for removing heavy material to 2000 plus for automotive finishes. Sandpaper with grit numbers between 60 and 320 and generally what we will be using.
Grit numbers under 100 can really damage a fine wood finish. This grit of sandpaper is used for stripping off heavy paints and varnishes. It may also be used to shape and smooth out rough lumber.
Medium grits, such as #120 and #150, are useful for removing lighter coats of old finishes and scratches. This grit is also used prior to painting.
Fine grits, such as #220, are frequently used for a final light sanding just before applying stain to the wood. The wood fibres tend to raise and this grit removes them without scratching to wood This grit is also used in between coats of paint. Hand sanding is generally used with this grit.
In woodworking, it is common practice to start with the appropriate grit and work your way up. From smoothing out rough lumber with a lower grit sandpaper and move to up to the final finer grits for preparing the finish and in between finishes.
The specific grit numbers you need vary by project and the material you’re sanding, so follow the abrasive product manufacturer’s recommendations.
The style of sandpaper one would choose is project dependent. One can hand sand with a wood block wrapped in a fine sandpaper for a final finish sanding. An orbital sander is very common when sanding down the lumber while building a project. Belt sanders are typically used for really rough lumber where a lot of material must be removed.
Sheet Sander vs Random Orbital Sander
Sheet sanders are not as powerful as the random orbital sanders. They are mostly used for finish sanding. Sheet sanders are also know as palm sanders. These sanders are relatively inexpensive Sheet sanders are good for projects with 90 degree angles. If you try to use a palm sander for rough wood, the paper will most likely tear and you will be sanding for a very very long time.
Random Orbital Sanders are much more powerful. They are used to sand the majority of your wood projects up to the finishing stages where one would then use hand sanding. Random Orbital Sanders are not good for 90 degree corners due to their round shape. These sanders are more expensive that the palm sanders but are well worth the investment.