Just wipe it down with a good CLP-type cleaner/lubricant. There are bunch out there, and they'll all work pretty well. If the slide rusted, it was bacause some of the sprayed-on finish had been scratched or chipped through. (The Tenifer treatment given to the bare metal is a surface hardening treatment that also resists rush, so rust must've had a good long time to do it's work. The colored finish is something like Cerakote. Wipe it down from time to time, and you'll not have more problems as the wipe down will keep the areas that CAN rust from rusting.
Keep a small rag (in a plastic bag) that is damp with the cleaner/lubricant tightly sealed in the glove box, and you can wipe it down from time to time.
As for avoiding WD-40, I don't think it's really as bad as so many folks claim, and I've never seen the sticky build-up that supposedly follows its continued use. I will note that nearly every "horror story" I've heard or read about as a result of using WD-40 on a gun was a second-hand story, from somebody who knew someone who knew about the problem or from a someone who knew a gunsmith who had problems cleaning, or from a grandfather who said he ruined a gun using it, etc., etc.
The WD-40 company itself strongly advertises its use as a preservative and lubricant, so I'm pretty sure unhappy customers could go after them if they had problems. But I've never heard of anyone doing it. (It is a great cleaner for stainless steel appliances. It is mostly mineral oil, with some other additives.)
Other lubes and solvents like Breakfree CLP, Ballistol, etc. are better for the type of rust-prevention process you need, but WD-40 is a good First-Aid step when a gun gets really wet, (dunked in a river crossing, etc.,) if that first-aid use (which will get all of the water out of nooks and crannies) is followed by a thorough cleaning and lubing, the gun will be almost like new.
or even better get you a CZ.
