Mike Keitz and Myke Predko wrote about potting stuff with Weldbond.

I once attended a good lecture by an IC packaging expert.
Main point seemed the battle against water permeating into
pretty much everything.

An ideal material would stick to silicon chip surfaces
(to prevent even a monomolecular layer of water forming
and enabling contact corrosion) but not stick to the mould
of a machine packaging 100s of chips/minute.

Epoxy rubber is good on the 1st point, plastic good on the 2nd.
Ceramic packs are good for hermeticity but at a cost.

Perhaps you could combine them by thinly coating the board in
epoxy rubber, then potting it in hard-setting epoxy resin?