Degrease the gas cylinder, brake cleaner works good. Twist a paper towel into a dowel shape and push it into the gas cylinder, the ring at the rear and the barrel path; to keep the paint out.

Hang the cylinder from the stacking swivel hole with a piece of coat hanger.

Paint it with any flat black spray paint, I use the store brand from Ace.

Bake it for an hour or so at 200-250.

It'll last for a long time, really cheap and easy to find the materials.

Jeff