Preventing rust on minor body damage
 

Wife backed her Impreza into a concrete post thing in a parking deck. Side of the rear bumper got scraped up a bit and then the very edge of the fender had the paint scraped off of it (probably a 2 inch long mark, just on the very edge of the fender).

It's very minor and the car is old enough that I don't care about fixing it properly. What is the easiest/cheapest fix to make sure that spot on the fender is sealed and does not rust? If it's touch up paint, that's fine, but it doesn't necessarily have to be.

Thanks in advance for the advice.

Touch up paint. Then wax it after it dries.