[Hallicrafters] Variable Capacitors needed

[Robert Nickels]

I should have included a link to these photos of the capacitors in 
question. 

http://smg.photobucket.com/albums/v652/ranickel/Grungy%20Capacitor/

These were taken before and after I initially cleaned them by soaking in 
the lime-rust remover "CLR".  That removed much of the grunge, but there 
was still arcing between the plates (370 volts) so I tried mechanical 
means,  running a folded strip of very fine grit sandpaper between each 
set of meshed plates.   This cured the arcing - for a while - but now 
it's back.    The appearance is slightly better than shown in the 
"after" pics here but since the plates look clean and there's no visible 
material in the airgap,  and since the arcing is occurring at various 
points depending on the angle of rotation,  I suspect there are surface 
irregularities that are causing the charge to build up and overcome the 
dielectric strength of the air, which is 3 × 10^6  V/m, or ~76 volts per 
1/1000th of an inch.   That means the specified plate spacing of .025" 
should withstand 1900 volts, whereas mine are breaking down at 1/4 that.

I think the idea of applying a thin conformal coating makes sense, but 
I'll glady accept further suggestions!

Thanks and 73,

Bob W9RAN