The growth of mould and mildew on a guitar case is common. Cases are often stored in basements or other areas where dark, damp conditions promote mould and mildew growth. Removal of these fungi is fairly simple, but also consider prevention of future growth. Most case materials are synthetic and resistant to fungal growth, but dirt and organic materials that collect on the surface will support mould and mildew formation.
Hard-case exterior cleaning tips
The exterior of most guitar cases is covered in non-porous vinyl material. Mold and mildew can be removed from the surface with warm water mixed with a mild detergent such as dish, laundry or car wash soap. You'll want to mix the solution with plenty of suds to avoid excess water while cleaning. After mixing, use a sponge or paper towels to pick up suds, and apply to the infested area, rubbing in circular strokes. Rinse the area with a clean, damp paper towel or sponge, and allow to dry thoroughly. To help prevent mould and mildew from forming on organic materials that may still be on the surface, spray the case with household window or surface cleaner, working it in with a paper towel, and removing any excess with a clean, dry paper towel. Store the case in an area that is free of moisture if possible, to prevent the likelihood of future growth. Placing the case in a sunny area for a few hours will also kill any remaining spores.
Soft-case exterior cleaning tips
Soft-cases for guitars are a popular case type, and are usually made from porous nylon or similar synthetic materials. Porous materials are more difficult to clean, as mould and mildew can get into the fibres of the material, but all is not lost. You can use the same sudsy soap and warm water solution as with hard-case cleaning, but apply the suds with a soft-bristle brush and rub in a circular motion. Remove the residue with a damp paper towel or sponge, and repeat as necessary. Spray the exterior with window or surface cleaner after the initial fungus removal, and wipe dry with a clean paper towel. As the case material is semi-porous, you'll need to take an extra step for drying it out. Mold and mildew spores cannot survive exposure to UV light, so placing the case in a sunny spot for a few hours will not only take care of the drying, but will kill embedded spores.
Case interior cleaning tips
Guitar case interiors are typically lined with soft, absorbent synthetic material such as felt, "plush" or fur-like material and other soft substances. The first step in cleaning soft, porous surfaces is the "dry" removal of as much mould and mildew as possible. This is best done outdoors to prevent spores from becoming airborne. If you own a vacuum cleaner with a HEPA filter, you can vacuum off most of the surface fungus. If you do not own a suitable vacuum, a stiff-bristle brush will loosen surface material. "Dry-foam" type automotive or home carpet cleaners work fine for most case interiors, but you should always test the cleaner in an inconspicuous area for colour fastness. Make sure you follow the instructions on the can, but most products of this type involve spraying the foam cleaner on the offending area, working it in with a soft-bristle brush and patting it dry with a clean, damp cloth or paper towel. Setting the open case in a sunny area will dry the interior and kill spores. If mould and mildew have caused the interior to smell musty, leaving it outdoors for a day or two will usually dissipate the odours. You may also sprinkle baking soda on the previously cleaned and dried interior, let it set for a day or two and vacuum the excess.