To dry out a cable which is partially air-spaced, get a cylinder of
dry air or nitrogen and allow the gas to seep through it under a
pressure of two or three times atmospheric. It may take several days
or more for a length of 100 metres. Measure capacitance at intervals
for stability.

It speeds things up if the cable can be warmed in an oven at about 60
degrees C. You don't want to melt the polyethylene.

The foregoing is based on a vague memory of an episode about 50 years
back.
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