These may be caused by water spotting, damp, or the use of plaster-of-Paris as a grain filler. Try rubbing the surface with a mixture of equal parts of linseed oil, turpentine, and vinegar; then clean off any greasiness that may remain by means of a swab of clean soft rag made fairly damp-not wet-with methylated spirit. Apply this lightly at first, then, as it becomes drier, press a little harder and finish in the direction of the grain.