Wonkee Donkee advise you to use a dry paint brush to sweep out the grooves in the mitre box to remove any dust. If a plastic mitre box is particularly clogged with sawdust, wash it in soapy water and allow to dry before using.
Wooden mitre boxes must be kept dry at all times otherwise the wood may swell.
Use a dry paintbrush to sweep out the saw guides and if there is a recess between the side walls and the base, pay particular attention to removing any sawdust that could be blocking the recess.
Beechwood mitre boxes and blocks should not be varnished or treated in any way, but simply kept in a dry environment.
Like plastic mitre boxes, those made from metal require little maintenance. Use a dry paintbrush to sweep out the saw guides and if required wash in soapy water and dry before using.
Storage
As with all tools, if you take care of your mitre box it will last longer and remain accurate for longer. Wooden mitre boxes and blocks should always be kept in dry conditions and preferably where there is no dust to clog the saw guides and where they cannot be damaged.
Plastic and metal mitre boxes should be stored where they are not likely to be damaged and, in the case of plastic mitre boxes, out of direct sunlight which can degrade plastic.