LDPE Caps for 15kv Transformer

Hi Coilers,

Anybody know if 120mills ( 3mm ) of low-density polyethelene wil be
sufficient dielectric thickness for
a 15kv  system?
Ive never made a rolled poly cap before, but want to get it right the first
time, as the LDPE sheet sure
aint' cheap ! Ime using a hospital-grade material, as the stuff at the
hardware seemed pretty poor quality.
It was speckled with little grains of 'crap' (????) embedded in it. (just
asking for trouble! ) 

My new Magnifier system will employ the series 'equidrive' system. The pair
of series caps will use
60mills each. I can increase the voltage on the transformer to around 17kv
with the variacs'  overwind.
Do you think they can handle it ? Should I change the variac tap to limit
voltage to 15kv?
I want these suckers to be 'bulletproof' !   :) 
I was told that LDPE can take 500V per 1/1000 inch. Does that sound right?