Does Bluing Prevent Rust?

Hot bluing is the current standard in gun bluing, as both it and rust bluing provide the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal.

Is cold bluing any good?

Cold blue (and Oxpho is probably the best) works pretty well on small items, like screwheads, or small isolated areas on a firearm where direct comparison to factory blue isn't apparent. Beyond those applications, it works reasonably well on matte blue guns for spot touch-up.

Does cold bluing protect metal?

Bluing is controlled rusting in a dark color; it's the red kind that causes an uproar . So, cold blue does protect the metal to an extent, from the red stuff.