I figured this site would be the best to answer this about rust on the frame. This is on my 6yr old truck I bought new and I don't live in the rust belt. This is the only place that looks like this. The other side looks good including the top, bottom and inside. Thought about a good sanding then some primer then paint. If I'm not on the right track to fix this, I would like to get the right way to fix it. Ive never dealt with rust on a vehicle.