Rick

Apart from the obvious causes of shorts, fine shorts which are difficult to see 
can be caused by over etching. This leaves overhanging copper which then breaks 
away during subsequent brushing prior to solder mask application. These slivers 
of copper are almost imposible to detect after solder mask without a current 
tracer. Pumice cleaning will only cause this problem in extreme cases of over 
etching and is therefore more reliable.

Inner layer shorts are more commonly due to photo tooling or registration errors 
unless they are plated and etched (buried vias) in which case the above could 
apply.

Good luck
-- 
Paul Gould
Teknacron Circuits Ltd
EMail [log in to unmask]