Repairing Bowed Guitar Necks
A bowed guitar neck is one which either has too much relief (upward bow) or not enough (dead flat or back bowed). While string tension creates relief naturally we need the instrument to fall within certain boundaries in order to maintain acceptable action and playability. When the truss rod can not or does not enable us to make such adjustments other means are necessary to correct the problem.
Necks With Excessive Relief (Bow)
If the instruments neck has too much relief (up bow) and it can not be adjusted via the truss rod, a refret and possible planeing will help in many cases. First we must consider how much mass would need to be removed in order to achieve a relatively flat fingerboard. Excessive planeing can lead to a board that is too thin on the ends or destruction of inlays. Another consideration is the neck itself. Certain necks have characteristics that make them poor candidates for repair. Weak, "mooshy" necks are not a candidate for neck straightening.
When refretting a neck with excessive bow I choose fret wire with a large enough fret tang to create a wedging effect and actually compress the fret slot. This tight fit helps to force the neck back or stiffen it depending on it's nature.
Picture of Fret Wire Tang
What about straightening a bowed neck via heating? The premises behind neck straightening by heat is to heat up the board enough to loosen the glue joint between fingerboard and neck, clamp the neck into it's new position and allow the glue joint to set up. In affect you are using the fingerboard as a splint.
Neck heating is not my chosen method of repair. When the neck has taken a "set" in this new position I prefer planeing as we physically change the shape of the board, not manipulate it and hope it will stay put. Apart from that, heating can cause frets to spring loose and brittle boards to crack.
Necks with No Relief (Dead Flat) or Back Bowed
While too much relief is probably more common some necks have the opposite problem...they're just too flat, or worse...back bowed. After loosening the truss rod to remove all tension on the neck these necks remain flat or back bowed even with string tension on them.
I see this far more often on maple necks than I do on mahogany necks though.
The first thing many may try is to use a heavier gauge string. The larger the gauge of string you use, the more tension is being placed on the neck and body. An increase in tension may be enough to pull relief into some of these necks and make them playable. Higher than usual action may also help in some cases.
If additional string tension is not enough to create the necessary relief, planeing may be an option depending on the severity of it's condition and type of fingerboard.
Squirrelly Necks
It's not twisted and the truss rod works but this thing ain't right! "I've had my guitar tech level the frets and adjust the rod but it buzzes like mad and plays awful and he can't figure out what's wrong." Ugh. These are real joys to behold. On occasion you will find necks that have what many refer to as an 'S Curve' or as I like to call it "a mind of their own". To me, this is a neck that does not 'relieve' or bow normally. The board takes on a bit of a roller coaster shape under tension that makes it difficult to adjust. These are most often seen on imported inexpensive guitars. If you pay close attention when adjusting the truss rod you will see some that really do some crazy stuff. Like what? Like, instead of straightening the neck, tightening the truss rod simply changes the point of greatest relief...instead of the dip being here it's there.
Even though you could hypothetically plane it's fingerboard perfectly flat or level the frets after the neck is clamped in a string tension simulating jig, these necks are normally just a pain to work with. And because they are most often seen on inexpensive instruments, time and effort must be kept to a minimum or the investment to repair may exceed the instruments value. When I know this is the case with a particular instrument I simply avoid them altogether.
Special Considerations and Unique Problems
There are those necks which create obstacles to our common methods of repair, they can create quite a challenge.
