Re: surface rust
Bluing on a steel gun is actually black iron (ferric) oxide. The gun appears some shade of blue in natural daylight because the oxide coating is very thin and the steel beneath is like a mirror. The thicker the coating the darker the finish. Ferric oxide is also called black rust. It is a very hard material; harder than soft steel or soft stainless steel.
Red-brown rust, that tends to grow on un-oiled blue steel surfaces, is Ferrous Oxide, which is a soft crumbly material that that can be removed (actually sheared off at surface level of the steel) with a soft Non-Magnetic stainless steel scouring pad sprayed with a material like WD-40, Kroil, PB Blaster, or any other light oil.
If one does not go crazy and rub too hard the above method removes most surface rust and does not damage the bluing.
Hope this is useful.