Leather will rust guns and knives--even ones with good quality steel & steel alloys. I have had very expensive motor boat propellers rust around salt water--I wash them w/ fresh water under high pressure, hand dry them or even occasionally use superfine wet sandpaper on them.
Usually, unless it's very poor quality steel, light surface rust getting worse can be prevented or arrested if it hasn't gotten out of hand. Commercially available Boeshield T-9 was designed by Boeing Aviation as a corrosion inhibitor for aircraft components and is excellent stuff for preventing rust--its like HD40 on mega steroids--we usually pack a spray can or two. Most marine shops stock it--good to have in the garage.
If rust has gone too far, then you're probably out of luck---like they say: 'rust never sleeps'.
Only metal I have that seems TRULY rust proof regardless of treatment is on knives made of dendritic cobalt steel--I always have one DCS knife on me when going on extended outdoor journeys. Dendritic colbalt steel knives have no magnetic signature, but I don't know of any guns made of that expensive steel--it might not be hard enough, although I'll bet David Boye, who's made some of my DCS knives, could manage it--but the FAA would hate it! H1 steel is the next best thing for rust resistance.
I wipe my guns, knives and other gear down if they've gotten wet and if they're wet from salt, including sweat, I carefully clean them with fresh water and in all cases, dry them. It only takes a couple minutes.
The most rust proof guns I have ever seen are Glocks. They have a hard time TRYING to make them rust--Glocks have survived some ridiculous torture tests. They have pulled Glocks out of lakes, creeks and rivers after more than a year, even soaked them in brine and while they might have some light oxidization (rust), the damn things still fired!
My every day carry stateside is an M&P (although I have several old and new Glocks, Sigs, HK, etc), but until 10 or 15 years have gone by and proven to me the M&P's are as durable as the Glocks-- if I'm taking ONLY one hand gun into the wilderness for a a long period, I hate to admit it, but it'd still be a Glock.
The M&P has never let me down and I like how it feels and shoots better than my Glock, but time is the true judge. I have never had a gun that I can shoot so accurately without fully sighting--I'm rather accurate by merely just fully extending my arm, looking down it as I point and then shooting with an M&P--very ergonomically friendly.
We'll see just how well the M&P's hold up over time, before I think about taking it deep into the middle of nowhere, which includes desolate ocean islands, with some people on them who have an intense dislike of Americans!
