We have found that good results are obtained from H3PO4 using a
two stage dehydration: 1) to bring 85% (Cap's "jug" grade) up to
about 97% (in half litre batches taking about 8 hrs) and then 2)
on the day of use, for about an additional 4 hrs in the reaction
vessel about to be attached to our VG common acid bath system.
We use the same system prior to loading the vessels into our
individual acid bath device. 

A different problem that we have found, and maybe Cap or Peter
can comment on this better than I, is that with too much pumping
the acid tends to polymerise reducing its efficiency. I
personally agree with Peter that relatively little is really
known about the chemistry involved.

Richard Corfield.