Bearbong as suggested use 4ought (0000) steelwool. WD40 is okay for this.Other then that I wouldn't recommend using WD40 on your gun.There are much better lubricants suited for firearms.Saturate the steelwool and lightly rub the spot.Don't go overboard rubbing or you'll remove what blue is left.For rust protection I use a sheep skin wipe saturated with RIG (Rust Inhibiting Grease). RIG is avialable at most gun shop and shooter supply wholesalers.After 30 plus years of owning firearms never had a rust problem. Been using RIG most of those years.It's inexpensive, and a little goes a long way.

Ruger Redhawk