Fear of Sanding

I learned another neat trick a while back from an auto-body guy related
to Chip's comment. This applies if you're repairing or sanding a
limited area (e.g. just the outboard 3 feet of wing). A common
problem is "managing" the size of the patch or sanded area and
especially the feathering to the untouched areas. ie. Since you
always want to make sure you remove the coarser grit scratches, you
need to make sure that each finer grit completely covers the area
covered by the prior grit. In addition to changing the bias on the 45
degree passes, you can manage the edges/feathering of the patch by
using successive layers of masking tape. You apply the tape in
"reverse order"; ie. you put the outside layer on first. Then, you
slightly overlap successive layers, working in toward the area you want
to contain. You need as many layers as grits.

So, if you are going to be doing 5 passes (400, 600, 800, 1000, 1200),
you need 5 layers of tape. After you do the 400 pass, you peel away
the inner-most row of tape, thus exposing another inch or so of
"interface area". When you do the 600 grit, this ensures that you
cover the entire 400 grit area plus "a little bit." Same with the
800, etc. (in reality, this is most critical for the 400, 600, and
800 grits; once you get beyond that, it's pretty painless to manage )
It's easier to do than it is to describe.