Success!
I’ve cracked the code! This is Thompson Enamel’s ground coat:
Also known throughout the Enamel on Steel – some insights article as Grip Coat and otherwise known as an enamel undercoat, this enamel, when applied as a base layer, will help enamels applied over the top to stick to the steel.
I had mentioned that WG Ball had a version yet despite my inklings that they also did one, I could never find it on the Thompson site. Until now!
I know I’ve said it before, but it’s the small things…