Looking after an instrument you built

An acoustic instrument under string tension is a structure being slowly loaded. The top bellies a little behind the bridge and dips a little in front of it; the neck takes a small amount of relief; the action rises. On a well-built instrument all three are small, they happen mostly in the first year or two, and then they stop.

This is why the setup you did the week you finished it is not the setup it will keep. A saddle lowered once at around a year is normal rather than a correction. What is *not* normal is movement that continues, accelerates, or is not symmetrical — and telling those apart is the whole skill.

Expected, once

Action rising, a saddle adjustment

Expected, seasonally

Action moving with humidity, both ways

Not expected

A joint moving, a crack, a rising bridge

Wood moves with moisture across the grain and barely along it, and an instrument is an assembly of pieces whose grain runs in different directions, glued rigidly together. That is why humidity swings do damage that a temperature swing does not: the parts want to move by different amounts in different directions and they are not free to.

A dry instrument shrinks. The top goes flat or slightly concave, the action drops, fret ends start to protrude past the edge of the fingerboard because the fingerboard shrank and the metal did not, and in the worst case the top splits. A wet one swells: the top bellies, the action rises, and the tone goes dull.

What to do about it:

• Measure rather than guess — A hygrometer in the room where the instrument lives, or in the case. Central heating in winter and a fire in a room are the two things that produce the dangerous end of the range, and neither is obvious without a number.
• Case humidification in a dry season — A case is a small controllable volume and an instrument on a wall is in the whole room. If the room runs dry in winter, keeping it cased with a humidifier is far more effective than trying to change the room.
• Protruding fret ends are a warning, not a fault — They are the first visible sign that an instrument is drying out, and they appear before a crack does. Filing them is treating the symptom; rehumidifying, slowly, is treating the cause, and the fret ends often go back.
• Bring a cold instrument up to temperature in its case — A cold surface in a warm room condenses moisture on the finish and on the wood underneath. Left in the case for an hour, it warms up gradually and does not.

• Nitrocellulose is soft, and reacts to things — It hardens and shrinks for years, which is where finish checking comes from, and it reacts with some rubbers and plastics — a guitar stand with the wrong padding will leave a permanent mark on the neck. Wipe it down after playing; hand acid dulls it.
• Oil and shellac are repairable, which is the point of them — French polish and oil finishes mark easily and can be touched in without refinishing a whole surface. If you chose one, you chose maintenance over durability deliberately.
• Polyurethane is durable and unrepairable — Nothing much marks it and nothing much fixes it either. A deep scratch stays.
• Avoid silicone polish on anything you may refinish — Silicone contaminates wood and causes fisheye in any future finish, and it is very difficult to remove. This matters more to a builder than to a player, and it is the one product to keep away from the workshop entirely.