Luthiers are pretty ingenious people. Always dreaming up new ways to improve their craft. I recently learned a very neat trick for sanding down tiny areas and repairing scratches. Here is the story.
Somehow I had put a rather deep scratch in this soft redwood top. I carefully filled it with thick super glue and let it dry hard. But how could I sand the superglue perfectly flat without damaging much of the surrounding finish?
Here is the secret weapon. A piece of 1/4 inch thick acrylic with the edges carefully smoothed so as not to scratch-to which a tiny piece of stick-on sand paper is placed. I learned that the sandpaper could be much smaller than even the strip shown in this photo. I chose 400 grit sand paper
Now you can gently sand the super-glue down, seeing exactly where you are sanding. The acrylic plate keeps your approach flat so you don't sand deeper on one side than another. The patch of sandpaper could have been a third as large.
Now I have sanded it down almost flat, but you can see the adjoining finish is now beginning to be scratched. Time for another idea,
By putting strips of scotch tape on either side of the sand paper you limit your sanding to the thickness of the scotch tape.
That got it down most perfectly flat. just a little more sanding and then a coat of dilute shellac.
That scratch is gone