Homebrew PCBs

I'm new to H_PCBs, but I've been a tinkerer with electronics since the
60s. I've worked for a few companies which had very good luck with
negative resist techniques, but I only had luck with one.  It was in
the late 70s, early 80s, I believe.  The name of the company supplying
the resist was Coval Industries out of Champaign, Ill.  You painted it
on, developed with water, could use incandescent or sunlight for
exposure, and was not terribly fussy.

The chemical was a rather thick green "paint".  I had one bottle of
this stuff, and could pretty much etch to my hearts content, using the
Chart-Pack (sp?) technique of stick on strips and pads on a drafting
vellum.  It (the resist) was very forgiving.  The trouble was, I
didn't have a good exposure box, and my layouts would fall apart
during exposure.  Also, being a negative process, you had to use the
etchant and resist twice to get one pc board.  So I wound up using the
first bottle learning good technique.

When I ordered a second bottle of this wonder chemical, it didn't
work.    
 
I called the company, and got the president of the company on the same
day the phone was to be pulled and company closed. Seems the process
quit working, and no one knew why.  I tried to buy the process, but he
wasn't interested.

Does anyone have any idea what this chemical might be, and how to
produce it?