Woodworkers could talk for days about the finer aspects of sanding.
The bottom line is that you use sandpaper from progressing from
coarser to finer. You just can't be impatient, or skip steps, and
still get a nice smooth finish.
Start with a coarse grit paper (like 80 or 100) to shape something
(like a pick). The paper has large grains of sand (the sand can
be different materials like garnet, or diamond, or kryptonite).
Each grain scratches off some of the material. Course paper makes
big scratches, wearing away the pick to the shape you want.
Next you switch to a medium grain (like 220). It has smaller grains
of sand and makes smaller scratches. You wear off the big scratches,
replacing them with smaller scratches. You have to get rid of all
of the big scratches. The medium paper can still "shape"
the pick, but it doesn't go as fast. You also need to worry about
loading the paper with dust. The paper stops working when it is
loaded, so get some fresh paper. Once you can't see any big scratches
among the small scratches you can move on to the next grit.
Do the same thing with the fine (400, and then 600). The plastic
should start looking shinier. Finer grits of paper are more likely
to load up, so change them frequently. Buffing is last and is basically
a very fine sanding, using the cotton (and buffing compound) as
If you do a poor job of sanding and move right to the finer paper
or to the buffer you will just have nice shiny scratches that still
sound bad. Sometimes I miss a scratch (it is a little hard to see
that white plastic). I'll only see it after buffing, so its back
to the medium paper.