The Rust Repair Kit is a consumable tool used to remove the rust from your vehicles. It is often used in conjunction with the Dent Repair Kit to achieve a smooth, even finish across your whole car.
They can be purchased from Kali-Gas for $49.99 each, and a total of two are stocked on the shelves per day. To use one, equip it using E, and click the body panel you wish to remove the rust from; the kit will be consumed, and you'll be left with one panel of your vehicle rust-free.
For especially rusty panels, you will need to reapply a layer of Clear Coat to make the color match the rest of the car, again.