Hi Mike, Rustoleum does not work well. Not very much pigment in it.
I use two types of black in an engraving. One for deeper cuts and background, and another type for shading and bulino light cuts.
There is information about blacks at the bottom of page link:
http://engravingschool.com/private/FAQ.htm
I haven't been using the hardware store type paints as the ones I tried didn't seem to have a lot pigment to the solvent ratio in them.
I use a product that way back used to be called "Background Antique" from the jewelry suppliers. The name of it has changed over years but it seems to be the same stuff. It is a flat black lacquer paint but with a lot of pigment. Paint it over the engraving with a brush, and while it is still wet, quickly wipe off the surface with a paper similar to newspaper. I use this paint in the deeper cuts and background. For fine shading use an oil base printers ink. Smear or dab this into the fine cuts with a finger, then wipe off the surface with the same type of paper. Dab it on carefully so you don't get in on your flat black paint areas. If the metal has a nice polish, wipe only in the direction of the final polish of the piece.
Here is a link to an oil based printers ink for shading cuts
https://www.dickblick.com/.../cranfield-traditional.../
Here is a link for the black lacquer with good amount of pigment.
https://www.riogrande.com/.../midas-...ound.../335400
Here are previous thread on the forum about the paint and inks:
http://engravingforum.com/showthread.php?t=10454
http://engravingforum.com/showthread.php?t=11634
http://engravingforum.com/showthread.php?t=11761
http://www.engravingforum.com/showthread.php?t=3150
http://engravingforum.com/showthread.php?t=6398