Roger, I have commented on our site, the keys are in sets of an octave 2 white mouldings and 1 black. they do not respond to cleaning as the type of plastic includes the hinge/pivot point and has to be flexible so this does mean the plastic is softer than the type used in the KN7000 and also much more prone to yellowing over time. My son's KN1500 had some broken keys so I had a chance to experiment with different ways of whitening without any form of success, the yellowing seems to go right through the key rather than just the first few thousands of an inch which could be cut away with cutting pastes.
Regards John.