We have an electronic / automotive (dashboard) product that is about
to be released, and the first production run will consist of
hand-built units.  In order to solidify and encapsulate everything, we
would like to be able to coat the constructed circuitry with a thick
layer of what is commonly referred to as potting compound.

We also regularly stock cold weld/epoxy compounds, but these are far
too expensive to coat an entire circuitboard in a thick layer.

My question is:  Does an appropriate, inexpensive and available
"filler" exist that would mix properly with cold welds or epoxies to
effectively create an electrically insulative coating for circuitry?

If so: what is it, where can I obtain some, what particular brand(s)
of epoxy or cold weld would work best with it, and what percentages of
each component (steel/hardner/filler, for example) would be
appropriate for an optimum mix?  Also, what ranges of mix percentages
would be considered acceptable?

We are open to, and in fact hoping for, a "filler" solution that is
made of some readily-available and obtainable substance, such as dirt
-- or ash, or something  similar.

If not: Please help me locate a source for the materials and supplies
I would need for the industry-standard potting process (or a
less-expensive alternative that I had not yet considered would be
acceptable as well).

I believe potash is what you seek.  It can be garnered simply by
burning wood in a pot.

If you own an ignited-vegetation hydraulic filtration device,
procrastinate the changing of the filter element for a fortnight or
so.  This should result in a residue that might function as
appropriate filler.

You can use sandblaster sand or crushed glass provided the filler is
dry. Put the filler in the circuit board box and pour mixed low
viscosity epoxy on top. You can even warm the sand first to speed cure
and increase flow.

Another alternative is Cabosil