Dave Plowman (News) used his keyboard to write :
Which would be effective, if you could get the tape to stick effectively to completely exclude air - usually it doesn't, so can I suggest a blob of silicon sealant perhaps? Another way would be to put a blob of the potting compound over the end - I assume you got some with the seals and glands?
The manufacturer used to seal the cut ends with something similar to a tar like substance I seem to remember.
The idea is to simply stop the white chalky substance - the insulation (aluminium oxide?) absorbing moisture from the air.