How to prevent rust on guns

Ballistol, the original CLP. Really most any modern CLP or gun oil will aid in preventing surface rust but environment is key. Keep your guns in a cabinet with a Golden Rod to keep humidity in check and you'll be fine.

Like many have already said. WD40 is not good. I always wipe my guns down with either Ballistol or Rem Oil.....both do a wonderful job. Also try to not store your guns in a very humid area if it can be helped.

What humidity level is best to store guns at?

Weird, I use WD-40 all the time. Never a rusty finger print or patina anywhere.

Evapo Rust from Harbor Freight works great to remove rust

I learned this trick while protecting my knife from getting rust. A thin coat of Vaseline and no humidity will ever go to the blade also the Vaseline will bot evaporate and stay on the surface long long time 

On my guns I use it but I'm being careful not to apply on Orings I only use it on the exterior, I'm not sure if the petrolatum is safe for the Orings so use this with caution.

With the heat and humidity I have here in FL I sweat a lot and if I leave a drop of sweat in any par of the gun it will rust in no time. Vaseline has been my secret solution to this problem!