Your car's paint is protected by a layer of colorless paint called "clear coat." Over time, this clear coat can wear off, even in small patches, leaving your paint exposed to the sun. If you detect the patches early, you might be able to repaint the clear coat only. If the clear coat has been off long enough that the base color is faded or damaged, you'll have a lot more work to do. Having a professional refinish the car is best if you have little experience painting cars.
Sand down the area where the clear coat is gone. Wet sanding works best, which means spraying a mist of water from a spray bottle on the surface and rubbing it with 2000 grit sandpaper. If you don't need to remove any paint, skip to Step 2.
Strip away the paint down to the bare metal if the paint itself has faded or been otherwise damaged due to sun exposure. In this case, you may need a power grinder with a sanding wheel. Depending on how many spots have damaged paint, you may be better off removing the entire panel's paint.
Cover all nearby parts of the car you don't want to get paint on. This will include the windows and all trim panels that aren't already painted. Use tarp or old towels for large areas and masking tape for smaller ones.
Apply a coat of urethane primer to the surface if you stripped it to the bare metal . Block this primer after applying it by running 400 grit sandpaper wrapped around a rigid block over it.
Paint over the primed area, doing your best to avoid painting over any areas that still have paint on them; if you covered the rest of the surface, this is less of a problem. Apply at least two coats in even strokes, waiting 30 to 90 minutes in between coats--this can vary depending on the paint. You should wait overnight after applying the last coat.
Apply at least three coats of clear coat over the surface, waiting 15 minutes in between coats. You should let the clear coat cure overnight before you wax or polish the car.
