Your car's paint is its first line of defense against rust, but the paint job is bound to collect an assortment of stone chips, door dings and scratches in day-to-day driving.
If you allow rust to establish a beachhead, it will eventually perforate the metal, according to an article in the current issue of the Hearst magazine Popular Mechanics, and then you are in for an expensive appointment at the body shop.The way to avoid this is to catch the problem before the metal is gone. It is possible to repair most minor chips in an hour or two, in the privacy of your driveway.
Don't even think about using anything except automotive primer and paints. Rust-Oleum is terrific for a rusty swing set, but it is not compatible with the solvents in the paint systems used on cars.
You will need primer and touchup paint in those little finger-nail polish bottles.
Begin by carefully removing flaky surface rust and loose paint, right down to bare metal. Use a sharp awl or knifepoint, but be careful not to enlarge the chipped area any more than you have to.
To stabilize the surface of the metal, use 3M's Rust Avenger, which will combine chemically with the lingering rust. Skipping this step is not recommended if you live anywhere it rains or they use salt on the roads.
Using an automotive primer-surfacer and a small brush, carefully prime the metal, again being careful to remain within the confines of the existing chip.
If you do get any paint on the shiny part of the car, wipe it off with paper toweling. It may be necessary, when you are finished, to use rubbing compound to buff off any residue. The finish won't suffer any permanent blemish, provided you are gentle with the compound. Don't get any compound on the primer.
Let the primer dry thoroughly. Overnight is best. The primer will shrink considerably as it dries, so another coat may be necessary.
What you are trying to do is fill the chip about halfway with primer. Then, using a bottle of touchup color and its built-in brush, cover the primer with a couple of coats, allowing plenty of drying-shrinking time.
Eventually, you will wind up with an almost invisible repair. If the scratch, chip or ding was large, the surface may be irregular, but it will be the right color and it won't rust.
You can improve on the appearance even further by very lightly wet-sanding with 600-grit paper to smooth out the surface. Just build the paint a little higher than the surroundings, and then sand carefully until the paint is smooth.