Wipe With Distilled Water
Two types of materials commonly used to build humidors are Spanish cedar or Mahogany, both for their ability to resist mold and fungus and retain humidity. Sometimes these materials are dried in a kiln before manufacturing. A humidor built of dried-out materials can absorb water, so wipe down the inside surfaces of your humidor with a cloth damp with distilled water and the humidifying system running. All potentially dry surfaces in the humidor should be re-wiped and allowed to calibrate before storing cigars.
Ensure the Hygrometer is Properly Calibrated
Before taking readings on your hygrometer -- the small, electronic device inside of the humidor that measures air humidity -- ensure its reading is accurate. Fill a bottlecap with salt and a small drop of water, not enough to dissolve the salt. Place the hygrometer and the salt-filled cap into an enclosed, air-tight container for six hours. When the time is up, the hygrometer should be at exactly 75 percent. If not, you can still use the hygrometer and account for its difference. For example, if it reads 78 percent after the test, assume the humidity in your humidor is 3 percent less than displayed.
Use a Sponge to Test Absorption
First use a damp, clean sponge -- no soap -- to wipe down the insides of the humidor. Then re-wet the sponge so it's damp and place it inside the humidor on top of a waterproof membrane, such as a piece of cellophane. Close the lid and let the water percolate overnight. If the sponge is dry, it means some areas of the humidor are still absorbing moisture, so repeat the process and let it sit overnight again. This is an important step in preparing your humidor before cigar storage begins.
Do Periodic Maintenence
Check your humidor periodically for hygrometer calibration, as well as mold or fungus growth inside. Do not place your humidor in damp areas such as a basement, or in areas with high temperature fluctuation. Always use distilled water when preparing, cleaning or refilling your humidor. Chlorine and other minerals found in tap water can be extremely damaging to cigars and the humidor's interior.