My old school method:
1. Wire brush and grind back paint 2X diameter of rust you see.
2. Use a rust reducer (phosphoric acid type) that will cause a reduction reaction to any remainung pits of oxide. It also etches the metal nicely to recieve the paint.
3. Paint with a water cured urethane (POR15). The silver is thick and is great as a "primer".
4. POR15 can be painted over with enamel, laquer or urethane. For outdoor use a topcoat is recommended. I'd use a chassis paint that matches your color scheme and paint the whole rocker panel (I've actually used a brush on some friends' cars for this.).
This site is for a proprietary brand of water cured urethane, but it applies to POR15.
http://hirschauto.com/miracle.htm