I use black toner powder for a copier mixed with vasaline.
About the same as Jerrows. The more powder you mix in the thicker the stuff gets,,and a little goes a long, long way.
You can buy the refill bottles of the stuff at most any office supply store or dept store.
Or off the NET.
Get the cheap stuff as you only care that it's black in color and not how well it'll reproduce a paper copy on your computer.
Don't use too much. Spread the stuff out with a toothbrush to cover the areas completely and evenly. That'll leave about the right amt on the metal surfaces. You shouldn't have to keep adding more marker with each imprint,,just spread what's there around again each time with the same brush. Only adding more after the layer gets very thin.
Other colors available as well.