Here at MayosMusic, in addition to offering great guitar, bass, ukulele, banjo and autoharp lessons, plus complete luthier services that include guitar, string instrument and amplifier repair/restorations, we are also a great resource service.
Acoustic guitars need proper care and maintenance to play and look their best (and maintain their value). Sometimes issues can come up that need to be addressed. So let's discuss some of those now.
Guitars are made of thin wood which is easily affected by humidity (moisture). When humidity increases, the moisture content of the wood rises, causing the wood to expand and swell. High humidity combined with high temperatures may cause glue joints to weaken and possibly open. Humidity can also damage finishes by allowing mildew or mold to grow on it, by making it milky and bubbled, by popping the finish off of the guitar, or by allowing metal parts to rust which can then stain the finish.
When humidity decreases, the moisture content of the wood lowers and if this happens too quickly, it can cause cracking and opening of joints as the wood shrinks and pulls apart. Wood shrinkage can also cause the frets to become loose.
Here are lists of possible issues of humidity related problems.
High temperatures can loosen glue joints, warp the neck (if the guitar is strung and tuned), and cause softening of the finish.
Low temperatures can cause checking and cracking of the finish, and even cracks in the wood.
I recommend storing your guitar in its case (hard case) when not being played. This will affect a more gradual change due to temperatures and humidity causing much less or even no permanent damage. The case will also help protect your instruments against physical damages and keeps it cleaner.
Lacquer Finishes: Often, you only need a warm, damp cloth to clean your guitar. For grimier guitars, a good quality guitar polish (NO Silicone based polishes) may be needed.
Polyurethane Finishes: These finishes are very tough and may be cleaned with most household cleaners (409, Simple Green, etc.), but test in a small unseen spot first like the back bottom lower bout edge.
First, always check mounting screws and nuts every time you change strings, as they can work loose from vibrations, temperature and humidity changes. Don't over tighten. Most enclosed machines have a tension adjustment screw at the end of the tuning knob. Adjust to minimizes slop and play, and to a medium soft tension when you turn the knob. Open gear tuners should be lubricated once or twice a year. Use a clean heavy oil or very light grease (one that does not separate or dry to a shellac like varnish), apply, cycle each key a full rotation of the string shaft (13 to 26 rotations of the knob, depending), then wipe clean the excess.
Different styles of playing require different types of strings. Different types of guitars require the correct type of string (for example, never put steel strings on a classical guitar).
Strings are held in place at the bridge base by the bridge pins. The pin slot must be facing forward to properly align the string on the bridge base. The ball end of the string must be pulled tightly against the inside of the top of the guitar before fully inserting the bridge pin. Do not hammer the pin into the bridge base. Your thumb pressure is all that is needed.
The guitar strap hooks on the End-pin. It is at the bottom of the guitar in the middle. Do not glue or hammer the endpin, instead twist slightly while pushing.
Most guitars have adjustable truss rods. Old Martins and some older Harmony, Kay, Stella, and other "makes" do not. Some modern and vintage guitars and basses (e.g., Rickenbacker) have double truss rods to adjust the bass and treble sides independently.
If the truss rod is too tight, the neck will back bow, which causes buzzing and "fret out." If the truss rod is too loose, then the neck will bow forward causing higher action. Adjusted correctly the neck will appear to be straight, but there still needs to be some measured relief. Truss rod adjustments should be made by a qualified Luthier or guitar tech.
A complete set-up and action adjustment should include:
The neck must have a good "neck angle" to the body of the guitar. Too much of a "flattened V" will cause very high string height. A neck-set will correct this problem.
Regarding guitar tops, some bellying of the top is normal. Too much bellying-up and you have a problem. Likely a bad internal brace or bridge plate.
All quality acoustic guitars are made from solid woods and are most often built in a controlled environment where the relative humidity is 45-55%. A guitar humidifier should be used when the relative humidity falls below 45%. Products such as the "Dampit Soundhole Humidifier" work very well. There are even digital ones now that give a clear readout of the level.
The guitar and it's fretted instrument family (Uke, Banjo, Mandolin, etc.) probably travels more than any other instrument in the world. Make an effort to protect your investment. Some recommended suggestions when traveling are:
High quality guitars with lacquer finishes can be adversely affected by the interaction with vinyl and synthetic straps. It is best to always remove the strap from your guitar after use and store separately.
A quality, durable and safe guitar stand is as important as a good hard shell case. Hercules makes a great line of quality and safe guitar stands that help prevent your instruments from falling off the stand.