The eight-hour rule for brass and copper
Copper and brass surfaces are extremely sensitive to oxidation. Once spotting appears on a plated surface there is no cosmetic fix.
Metal Finishing
Sprayable patinas: metallic finishes without an electroplating line
Brass, copper, nickel, silver and gold reached with a spray gun rather than a plating tank. Useful for short runs, sampling, and any piece where plating is more process than the job deserves.
Rust before you even start: preparing iron for coating
Iron picks up surface rust in storage and between processes. Coating over it traps the problem underneath, and it will come through.
Why NC lacquer fails on iron
NC is the default clear coat in most furniture workshops, so it gets reached for on metal too. It is the wrong choice twice over.