My 30 year old house has original cedar panelling on some walls and ceilings. The ceilings in particular are covered in grey blotches roughly fingerprint size, which looks like maybe sun damage. The ceiling timber looks just like plain dressed cedar without any finish coat. If I spot sand (or even rub with a pot scrubber) then oil, the sanded blotches, which are now roughened, turn darker than the unsanded timber. I tried an oxalic acid cleaner which reduces the level of blotchinesss but its a really messy job and some greying remains, probably because these cleaners are really designed for external, flat decks which can be hard scrubbed and pressure jet cleaned, not practical on a celing. Has anyone any idea what the greying may be and how to remedy it, preferably without sanding the whole ceiling ?