Humidity and temperature
For a long, happy life for your guitar, protection from extremes of temperature and humidity is essential. No matter how well-seasoned, wood will always seek an equilibrium based on the humidity and temperature of the surrounding environment. Put simply, in dry conditions, the wood in your guitar will shrink and possibly crack; in wet conditions, wood will swell and possibly ripple and break glue joints. For the best results with your instrument, keep it in an environment where the temperature and humidity are as close as possible to the conditions in which it was built.
I build in a temperature- and humidity-controlled shop, usually at 20 degrees C (68 degrees F) and 50% relative humidity. Variations in both temperature and humidity will affect the wood in your instrument, but of the two relative humidity is the most important to monitor. Guitars kept between 45% and 55% relative humidity will be very happy, but if the humidity drops below 40% or above 60%, you should take action. Similarly, temperatures below 10 C (50 F) or above 27 C (80 F) are cause for concern. However, if the relative humidity is still within the safety range at those temperatures (and even beyond), everything will probably be all right. Watch the relative humidity!
Maintaining guitars in dry environments is relatively simple, since it is generally easy to add moisture to the air surrounding the guitar. In-the-soundhole humidifiers are helpful, but they are a less-than-perfect solution, since their effect is primarily on the inside surfaces of the guitar. The neck/fingerboard/head and the outside of the guitar may still be very dry. In-the-soundhole humidifiers are also notoriously difficult to regulate.
It's much better if all the air surrounding the instrument can be kept at the proper relative humidity. I do that in my own home (which is often very dry, particularly in winter) by devoting a corner of a closet to guitar storage. I've lined the closet with plastic sheeting and have installed a small humidifier inside it. The guitars are on a wooden rack just above the humidifier, unless I have them out for practice or demonstration, in which case they might be away from the controlled environment for a few hours or perhaps overnight. In the dry winter months I add water perhaps once a month. After a bit of trial and error with the settings, I haven't had to make an adjustment to the humidifier itself in years. It's about the simplest possible setup, but it works extremely well, and I've had no humidity problems with my guitars.
In high-humidity environments maintaining the proper relative humidity for the guitar is somewhat more difficult, since it is more difficult to remove water from the air than it is to add it. However, a closet or a room with a small air conditioning unit is a relatively simple solution. And as long as the guitar is away from the controlled environment for only relatively short periods, it will do well, even in very humid conditions.
Maintaining the finish
While French-polished shellac finishes are acoustically right, they are also somewhat fragile. Nevertheless, they can have a long life if properly cared for.
Water and spilled alcoholic drinks will damage a shellac finish, but the primary enemies seem to be perspiration and body oils. It is difficult to stop perspiring, particularly in hot climates, but a few simple procedures can help preserve the finish indefinitely.
1. Avoid contact of your right forearm with the side of the guitar by wearing a sleeve or some other arm covering. If need be, this can be as simple as a sock with the toe cut out, although most players will understandably want something a bit more elegant. If you put an armrest on your guitar, you'll avoid the need for an arm covering altogether.
2. Avoid contact between the back of the guitar and your stomach, perhaps with a cloth or some form of bib.
3. Wipe the guitar thoroughly with a clean cloth after every playing session. This is particularly important on the neck, which may need to be wiped clean several times during a playing session.
4. Wax the guitar with a carnauba-based polish at regularly intervals, particularly on the sides, back, and neck. More wax is not necessarily better, so this need not be done on a daily or weekly basis, but it is important that there always be a thin wax protective layer on the guitar, particularly on the areas of the guitar that receive steady wear.
5. Clean and renew your cloths, sleeves, and bibs regularly. It is important that they be as free of perspiration and body oils as possible.
If you play your guitar a lot, in spite of your best efforts the finish may begin to show wear, and in that case it may need to be refinished. Shellac refinishes easily, so this need not be a difficult or expensive task, so long as the wear has not exposed bare wood. I offer the first refinishing free to my customers, but I ask that you pay all shipping costs, so it may be more economical for you if the refinishing is done locally.
Lacquer is significantly more durable than shellac, and I now use a lacquer finish on the back and sides of most of my instruments. Maintenance is simple: regular cleaning with a soft cloth, plus quarterly or semi-annual applications of carnauba wax. Lacquer isn't indestructible, however, and treating it with the care you would give shellac will greatly extend its life and beauty.
I use an oil finish on the neck, fingerboard, armrest, and bridge of my instruments. Regular cleaning with a soft cloth is important, as is periodic renewal of the oil. Renewing the oil is particularly important when the instrument is new. I include a small bottle of oil with my instruments and can supply more upon request. Apply the oil with a soft cloth and let dry overnight before use. Usually only a few drops of oil are required. When re-oiling the bridge, use a very mild masking tape applied around the bridge to shield the top from the oil-filled cloth. Any oil that does get on the top will not hurt the finish and can be removed with a clean cloth.
12-hole tie blocks allow a simple and quick way of fastening the strings to the bridge. However, it is essential that a small overhand knot be tied in the ends of the treble strings so that when brought up to tension they do not pull out, whip around, and damage the guitar top.
Other maintenance tasks
If necessary, the fingerboard and frets can be cleaned and buffed lightly with very fine steel wool. Ordinary hardware store steel wool is probably not suitable, since it will contain oil from the manufacturing process. Look instead for an oil-free steel wool such as woodfinishers use. If you are unable to find something suitable, write and I will send you a small piece, enough to last you many years.
Tuners: the gears may need a very small amount of lubricant once in a great while. Dry, powdered graphite is ideal, but if not used in excess, simple vaseline will work fine.