Many motorists will tell you that it's ultimately under the hood what counts, but let's face it: appearance matters. Whether you want your car to be the envy of the neighborhood or you're planning on selling it to a private buyer, the aesthetics of your vehicle affect how others perceive it - and the driver. You might think that once an older car has developed rust, it'll be there for good. However, with a little elbow grease, you can get your older vehicle looking like new.
Before you start, get a breathing mask, eye glasses and gloves. This might seem excessive, but there will be paint and rust particles everywhere, so you need to take proper precautions. Similarly, you need to protect your vehicle - use a tarp sealed with painter's tape to section off the part of the car you'll be working on. You'll also have to "mask" the parts of the car that you don't want to get paint on. Don't be cheap and use newspaper, the paint will go right through. There's actually material made for this very purpose called masking paper that you should invest in.
The idea here is to remove the rust, and any paint that comes off with it. You'll then repaint the vehicle in the affected areas so it looks like new. Obviously, you'll need paint that matches your vehicle's current color.
To remove the rust, use a dual-action sander, starting at 80 grit and working your way up to 150. For the really tough rust, you might need a metal grinding wheel. Be careful with this, as you can do some damage to the vehicle if you hit more than just rust. There are also rust-removing acids that can work well for some of the tinier particles that might get left behind. If you make any dents, you can use automotive body filler to even out the bumps.
Apply the paint primer to the vehicle, spraying even thin coats on the affected area. Then use wet sandpaper on the area before applying the paint. Be sure not to go heavy on the paint at first, start with a light coat and then apply more until you get the desired look.
From there, it's simply a matter of buffing the paint so that it blends and letting the work dry.