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Here's a link to a decent show and tell about four different solutions: GunTech : Cold Bluing Touch-Ups - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools

I buy...

Hmm, good questions. If you only want a grayish surface I'd suggest some warmed, not hot, apple cider vinegar. Although any vinegar should work. This is a weaker acid, so you won't have problems...