Do a web search on "metal spraying" and you'll find your answer. It's not magic, or rocket science. They use the time old method of spraying molten alum thru a special spray gun. The molten alum solidifies on the surface to form a skin. You can do this with almost any metal, and even some ceramics, it's been around for ages. I've been toying with the idea for coating fuselage and wing plugs so I can put a realistic surface on it for moulding (Flite Metal etc is too thin to be able to polish the orange peel out of it left behind by the adhesive) but it's hard to find a shop around here who's willing to do such a small job. BTW, sculptors have been using it for ages to coat plaster sculptures so they can polish them up to look like they were made from metal, sometimes they spray bronze, copper or alum, or even stainless steel.