Oxpho Blue, sold by Brownell's. It is by far the most durable cold blue I have used. The prep work isn't real demanding, and you can apply it with steel wool pads to blend and get an even finish that is near impossible with other cold blues. No special tools or boiling tanks are needed. The harder you rub it on with the fine steel wool, the better. The same amount of rubbing with steel wool would remove other cold blues.
It is no substitute for a nice slow rust bluing job on a better gun. But it can certainly improve the appearance of a cheap gun that is in rough shape. The Brownell's Gunsmith Kinks books contain numerous testimonials and suggestions for prep and application techniques.
