Most dartboards that appear to be horsehair are actually made of sisal, a fibre from an Agave plant, although very old dartboards may contain horsehair. Bristle dartboards, no matter what material from which they are made, can be easily damaged if they are stored in a hot or moist place. If you are looking to restore an old dartboard, examine it closely first for moisture damage. Bubbles in the surface indicate such damage and may not be repairable.
Vacuum the surface of the dartboard to remove any dust or debris.
Use a soft but stuff brush to remove embedded dust and dirt from the surface.
Examine the dartboard for moisture damage, including mildew and mould. If you find mould or mildew, your board may be damaged beyond repair. However, if it is just mould found on the surface, you may be able to save it.
Take a mouldy dartboard outside so that mould spores are not spread around your home.
Use a can of compressed air to blow the mould and mildew spores, as well as any other dust, out of the board.
Leave the dartboard in direct sunlight for a day or two to kill the mould or mildew. Bring it into the garage overnight so that dew or frost does not settle on it. Sunlight may cause the dartboard to fade slightly, but this is better than having mould or mildew on it.
Dampen a clean cloth with wood polish and gently wipe it on the surface of the dartboard. Do not soak the board with the polish.
Allow it to dry thoroughly, at least overnight, before using.
Clean it again if necessary the next day.