I use the blue chalkboard chalk for inletting metal the into wood. I coat the wood with the chalk then rub a light coat of very thin oil on the metal. When you press the metal into the wood you see the spots that need relieving by the way the oil darkens the blue chalk. I was shown this trick by an old tool and die maker who built m/l rifles.

For metal to metal I still favor the lamp soot.

SRH

May God bless America and those who defend her.