Check this out

We had silver paint at IBM 50 years ago. There is nothing new there. I would question how much current it will carry tho. We only used it to repair CCROS cards. (Printed circuits on a paper IBM card used as ROM)
It was milliamps if not micro amps.

I doubt this will ever be a practical way to run line voltage and low voltage would require a lot of current.

When you draw your plug on the wall, do you need to draw a cover, too?

Agreed. There's no practical application for it in our realm. Just thought it be a chuckle to share

Used to be able to buy something like this to repair damage to printed-on elements of car rear window heaters. If it worked, (I never tried it), it must have taken several amps.

I see nothing revolutionary ... painters have been trying to do electrical work for ages.

On a more serious note, I can see something like this being used- either for static control, or even to create equipotential planes.

This stuff has been around for ages as "silver print", used for repairing damaged PC board traces, old time alarm window foil, etc. Also as mentioned a repair material for auto defroster grids.

http://www.chemtronics.com/products/product.asp?r=1&m=2&id=7

I question the statement that it can used on metal. How would that make a circuit. The entire metal plate would short everything out. It would need a coating of sort not bare metal.

..the consistency of Marmite..

'Marmite' is a brand of ..er.. sandwich spread [?], made from hydrolysed yeast- a by product of brewing beer. Very high vitamin B content. Powerful stuff, very salty and an aquired British taste, although half the pop. can't stand it! Also sold in New Zealand and OZ as a slightly different formula.

Isn't it also similar to Japanese miso?

As others have said, this is very old news. The application I'm familiar with it being used in though is field radio antennas. You'd paint the antenna on whatever and then attach an included lead to it.

Doesn't mercury predate all that?