RE: Birchwood/Casey perma blue
Oxpho-blue is a cold blue, of course. Is is a surface coating, like other cold blues. Blue Wonder is entirly different, I believe it is a photo chemical reaction to a micronized metal coating that does penetrate ferrous metals. Doesn't rub off, like cold blues, and does provide a very good degree of corrosion protection. Corrosion protection is higher than hot blueing, because the finish is not a layer of oxidized metal.