Some months ago I put shims under my second fret where the strings had worn little U-shaped indentations.
At first they worked OK. But as time went on I found it harder and harder keep them adjusted, and this morning it was impossible. I think the reason may be this. If you shim the bottom of the U up to the the correct height, then the string will buzz against the sides of the U when if the plane of vibration happens to be parallel to the fret instead of perpendicular. So, apparently shims may useful with unworn or lightly worn frets, but they are not a solution to deep gouges. Printer paper, 3x5 cards, and business cards all shim with no appreciable loss of sustain. To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
