PVC Water Absorption: fallacy?

From:  Edward V. Phillips [SMTP:ed-at-alumni.caltech.edu]
Sent:  Friday, May 29, 1998 3:46 PM
To:  tesla-at-pupman-dot-com
Subject:  Re:  PVC Water Absorption: fallacy?

"Many people recommend that PVC coilforms should be varnished inside and out,
in order to seal them against water ingress.

Having spent an hour or so in our library with various materials handbooks,
I see no mention  of any significant water absorption of any of the rigid
PVCs which are encountered. It is a very good plastic, comparable with HDPE
(rigid polythene, alkathene). The volume resistivity is not affected by
immersion in water, and the surface resistivity is only marginally worsened
(same is true for most amterials: even touching the test sample will cause
an OM change in ohms per square). Nylon and PTFE do suffer, but pretty well
all of the thermoplastics are not prone to absorption. They are impermeable."

	Have never understood these recommendations myself.  I have made
a number of very successful Van de Graaf generators using PVC pipe
for the insulating columns, and have never seen ANY evidence of 
moisture absorption.  Because of their minute current output, these
gadgets should be order of magnitudes more sensitive to moisture
absorption and resultant leakage.  In fact, I have often cleaned
the columns with soap and water after they had attracted enough
dust and grime to begin to break down regularly.  After washingt
washing they are then rinsed thoroughly with TAP WATER!!! (Too
cheap and lazy to go get distilled water.), and dried with a
dish towel.  This is followed by a few minutes of hair dryer
attention, enough to evaporate moisture on the surface but
surely not enough to drive out any absorbed water.

For what it's worth,