This is how I do it.
I use fine 000 grade wire wool for removing the rust off the gun with some gun oil, WD40 will be fine.
The idea is to remove the rust without damaging the surrounding blueing. Hopefully the rust will be just on the surface and hasn't pitted.
Next I degrease the area. I use turpentine but white spirit would be fine, you could instead smear washing up liquid over it and wipe clean with a damp cloth instead of using solvent(just make sure it's dry). I've even know fingerprint oil leave a mark if not removed.
I then rub the blueing solution over the metal with a lint