I would suggest cleaning lightly with Q-tips that are wetted but not saturated in a dilute non-ionic detergent. Apply light pressure. Do not saturate the Q-tip so the cleaner gets onto the felt. Wipe with clean water using a wet but not completely saturated Q-tip, cotton swab or un-dyed, lint free cloth and dry immediately. First, if possible, test the pressure and dilution in the least visible area; work one small section at a time and "do no harm;" therefore use the lightest pressure and weakest dilution possible to remove surface dirt. It may facilitate cleaning if you mask the felt (perhaps using a mylar "donut" with a slit cut along the radius and slipped in over the felt to protect it from the liquid on the Q-tip; don't use tape or anything that will leave a residue). I suggest the following for sources of non-ionic detergent and, possibly, other cleaning materials: http://webcache.googleusercontent.com/search?q=cache:ZvBqyhhWQvQJ:www.nps.gov/museum/publications/conserveogram/10-02.pdf+&cd=1&hl=en&ct=clnk&gl=us.