Guitar repair question - nut shim

Scall,
Shimming a nut will raise all of your string equally by the
amount of the shim. It will also have an impact on your tone...perhaps
not perceptible to you, but it is there...here's why. The string of a
guitar is designed to vibrate between two fulcrum that are properly
coupled at two points. The first is on the body at the saddle (bridge),
the second is at the end of the neck either as a zero fret or as a nut.
Any material placed in-between the nut or saddle will slightly uncouple
the fulcrum which means that part of your string vibration will be
transferred to the space between the shim and the guitar and betwen the
shim and the fulcrum (like the ham in a burger ;-) This uncoupling
(however slight) means that your sustain will suffer (and thus your
tone will change). The better solution is always: Go to a luthier/tech
and have a new nut properly cut...and here's a good chance to
experiment with new nut materials (like bone or Tusq). If you need to
use a shim, my preference by far would be to use a hardwood veneer,
preferably one that is similar to the wood in your neck (likely mahog).
I would also fix the shim with a little bit of Hide glue to maximize
transfer (rather than leave it floating). I would then fix the nut to
the shim with a drop or two of Cyano.

It is evident that your teacher thinks your nut action is too
low...here how to measure it properly. Place a capo (or finger) on the
THIRD fret. Now measure the gap between the bottom of each string and
the top of the FIRST fret. There should be a very, very slight gap
(about the thickness of a hair...a little more on the bass side than
the treble). If the string is actually resting on the first fret, the
slot may need to be raised...if all the strings are resting on the
first fret, then the nut needs to be raised. In your case, if only one
string is causing the problem, raising the slot is likely a better
solution (though not as good as a new nut). It is rather important
before you do anything to be certain that this is the problem...if you
raise the nut slot and the problem is actually a sprung fret on the
second (look at the second fret on the bass side...now press down on
the top...does the fret move up and down (even slightly)...if so, your
nut is fine and you need to tack down the fret (by seeping in a little
viscuous cyano acrylic glue).

The nut needs to be cut rather precisely with things like back angle,
width, and the shape of the seat all being customized to the string and
headstock angle and the radius of the fingerboard. This is why there
are specially made nut slotting files that only cut on the edge and
that come in precise thicknesses to match the string gauge. You can cut
it with other implements of destructions, but your success rate will be
hit and miss with these McGyvered solutions unless you have a lot of
experience. For this reason, I would not fill it as mentioned in other
posts...you'd want to smooth the seat and properly shape it after
filling, and without the proper files, this can be your downfall.

Rather than go this route, I would suggest that you simply buy a roll
of plumbing tape (a.k.a Teflon tape). This is the white tape that is
designed to go around the thread of water pipes ensure leak free seals.
If you fold a small hunk over a few times, you can fit it in the slot
and tune up. The pressure of the string will shape the seat and
compress the material...the teflon is very slippery and will discourage
binding (the symptoms of a badly cut nut slot). This is 100%
non-destructive and about as effective in slightly raising a slot that
was cut too deep...you simply need to loosen the offending string,
place the tape in the slot and replace the string...it is ready for
tuning and testing...add more tape to taste. Yes, this also uncouples
the string slightly, but not near as much as a shim.

If you do insist on shimming your nut (which is acceptable, but not
optimal), you need to be a little careful. Most makers install the nut
before they finish, and so, here are a few cautions when messing with
the nut. First: Remove all the strings. Next, score the headstock side
of the nut with a sharp knife along the seam of the nut to break any
bond with the finish...otherwise, you will likely take a big hunk of
finish with your nut. To remove the nut, place a small wooden block on
the fingerboard, and lay it against the nut. Using the palm of your
hand, give the block a slight rap towards the machine heads...watch
out, the nut sometimes flies off. DO NOT sand or file the seat without
the proper nut seating files...you will likely need to do some cleaning
(most nuts are held with a small dab of glue and this will need to be
carefully removed before proceeding...go slow). You really want to
avoid taking anything off the exposed end of the fret board...bringing
you nut closer to the bridge will invariably mess up your intonation as
you alter the scale length.

There is little doubt that the Low E (the fat wound one) is
problematic. The fatter the string, the more space it need to vibrate,
so it is essential that the bass side have slightly higher action than
the treble. It is also essential the the relief is not too excessive
(causing a big dip in the middle of the neck (~7th fret) and shallow
sections from the 1st to 3rd and 12th to 14th frets). However, you
should also be aware that the Low E will buzz regardless of your string
height if you 1) have a very strong downwards attack (causing it to
vibrate really hard when compared to the other strings), 2) Have an old
tired string (true of all wound strings...as the winding deteriorates,
the string will buzz) or 3) have sloppy fretting technique.

Diagnostics on stringed instruments are not always evident and a
solution sometimes appear complete unrelated to the symptoms. All do
respect to your teacher, my advice would be spend the $45 to bring it
in for a proper setup...mention your buzzing and stick around...your
axe should play better.

Cheers, CS