I think many of the browning solutions that are on the market today have changed their formulas, because of the toxicity of some of the ingredients in the original formulas. I know for years I used a product called Ol' Thunder, that was the best in our area. It put down an even, very fine grained, coating that would wear like iron. The product is still available I think, but the last time I used it, it didn't work half as well as in the past. It took many more coats, and came out quite blotchy. Another builder told me the original formula had some sort of mercury based ingredient in it, that had to be removed.
A friend that owns a metal fabrication shop, uses some sort of metal treatment, that is designed to blacken iron, and steel, but when treated like most cold browns, it put down a beautiful reddish brown, with a very fine grain, and good wearing characteristics. He gets it from a chemical supplier that specializes in metal treatments for the industry.
Hungry Horse