Coating Interior Surfaces

Q. We have a requirement to apply a liquid dry lubricant product to the inside of an aluminum gun part that is one-inch ID. This coating is a dark gray moly-lube and more difficult to apply than lacquer. Our problem is to find a spray tip or system that will work in this confining space and not result in a course finish. One spray gun supplier tried their 360-degree spray extension in their lab, but we're not impressed and did not recommend buying it. We have the mechanism to advance an extension into the part, but can’t find a tip that will perform. W. M.

A. I’ve had extensive experience coating interior surfaces, but nothing that small in diameter. Unfortunately, I have no magic spray gun to recommend for this application. What you are dealing with here is Izzo’s First Law: “The innate perversity of inanimate objects.”

There are a couple things you can try. If you are using a 360º nozzle that sprays a disc shaped pattern perpendicular to the axis, try using a 360º nozzle that sprays a hollow cone at a 45º angle to the axis. Another approach is using a powdered version of the dry lubricant. Preheat the part and dip it into a fluidized bed of the material. I can’t think of anything other than those suggestions. Perhaps a reader can help.