For a collectable, I would take the gun down while wearing nitrile gloves to prevent contamination with finger oils and salt. Remove the slide and thoroughly degrease the rusted portion with virgin solvent. Gun Scrubber or no-residue spray solvents work. I prefer to hook a stainless wire through something like that and lower it into a jar with enough virgin naphtha in it to cover the workpiece. (Naphtha is sold by the gallon at Lowe's.) I leave it for a day or so to let it pull all oil from the rust. That does a gentle job and protects it from further rust while it's submerged.
The next day, remove the part from the solvent by the wire and hang it until it dries. Next, lower it into boiling distilled water and keep it suspended there for about 15 minutes. That will convert the red rust to black magnetite and also loosen it so it is easier to remove away. When you pull it out, the heat should dry it fast. Nonetheless, you will want a clean rag handy so you can grasp the slide and shake extra water off and out of crevices to get it to dry faster. Drying hot like that makes thin blue oxide rather than red rust on bare steel.
Next, do something that will make you want to cringe. Submerge it for a day in WD-40 (Auto suppliers have it by the gallon). Alternately, use the water-displacing oil sold by Brownells for Parkerizing. Either one will draw water out from the front sight tenon or any other place it might have gone into a crack.
After a day of soaking, handle the slide with Nitrile gloves to avoid fingerprint contamination. Start wiping with a cloth patch to pick up the loose rust. It's just abrasive enough that you want the soft cloth to pick up what it can gently. Follow that by light scrubbing with an old toothbrush, then another patch . Finally, use a coarse brown paper towel to get the rest.
At this point, if you used WD-40, you want to remove it or it gets tacky over time. Put it back in the naphtha to accomplish that. After a day, pull it out and let it dry, then hose it down with LPS2 or Birchwood Casey Sheath or some other rust inhibiting surface lube. Put the gun back together.
For storage, you want something 100% synthetic to avoid holding moisture. I've used kitchen plastic wrap around guns to be stored in their original cardboard boxes before, having see the cardboard initiate rust. That's actually worked pretty well. You have to wipe the gun off with fresh rust preservative after removing the plastic, as it leaves a pattern in the rust inhibitor, but no damage is done to the gun. A polypropylene hiker's sock liner might do OK. Just test whatever you use with the preservative oil you want to use first, so you know it won't soften and adhere to the gun.
It occurs to me that, after boiling, the loose oxides could probably be removed in a large ultrasonic cleaner without incurring abrasion of any kind, but I've not tried that personally. Anyone up for the experiment should report on it.
I should also mention that I've encountered rust before that did not seem to want to convert by boiling, though I expect grease saturation that did not leech out completely and the sheer thickness of the deposits may have been responsible.
