At the fab shop I work in, we use a lot of patina finishes. Mainly they are Sculpt Nouveau products. The Birchwood Casey Gun Blue is very similar to Sculpt Nouveau Slate Black Patina. A beautiful rust patina can be achieved with Sculpt Nouveau Japanese Brown applied to a hot metal surface. Here are a few examples of patina finishes that we have done.
Japanese Brown, Hot, Satin Clear
Deep Brown, Cold, Satin Clear
Slate Black on Copper, Cold, Burnished, Satin Clear
Blended Mix, Cold, No Clear
Blended Mix, Cold, taped off the patina area first, cleared the deck, removed masking, patina applied, Satin clear over the entire surface.
Skateboard deck made with 1/8" mild steel.