Homebrew PCBs

Does anyone here have experience and suggestions for low-leakage
coatings? I've got some circuits in mind that in the past I've done
dead-bug style to avoid surface leakage on a PCB. Is there a good not
to expensive low leakage coating?

I'm thinking of an input with an impedance in the rage of 100M to 10G.
Yes, I even wash the parts themselves, I've been either leaving them
uncoated or coating them with paraffin. Something I got from a book on
static electricity machines, mix a bit of lighter fluid and wax and
paint it on. The lighter fluid (not butane, I mean the stuff that goes
in those old metal lighters) evaporates leaving a very thin even
coating of wax.

Steve