Rust SUCKS!! If this rust is in the DOG LEG, it most likely came thur from the back side, most likely from fine dirt laying in the joint where the inner body panels meet. Can't really tell from pic because working from I-phone. First, if I am right , you need to wash all dirt out from front and back sides before starting any work. Blow or vac dry. Next, grind any lose rust off or wire brush or wire wheel. Don't know what you have to work with. Next, get some type of rust inhibitor - Rust X- or what ever your parts store carrys. Treat as directed. At this point I would apply a good epoxy primer. PPG makes, I my opinion one of the best. DP is that product. Comes in different colors, but is expensive. Any epoxy will do, just make sure it is a two part , which means you will need to add a harder. You can brush on if you don't have a spray system. That should hold till your ready to go further. You can paint over the epoxy and try to get a close match to you paint if your not getting the rest of the body work done for a few months. Hope this was a help.