1.Etch primer is used for area's that have an adhesion problem or risk. It is acid that etches or eats its way into metal. You apply a very thin coat of it on bare metal area's. A common misconception is that it is good for corrosion resistance. Not true. It provides adhesion and that is it.
2. The first thing someone suggests is epoxy. Epoxy is great for corrosion resistance and has great adhesion qualities. The biggest drawback of epoxy is it sands like crap, and personally it is NOT meant to even be sanded and is non sanding!!. The mention of DP40 is a great product, but it DOES NOT sand well, to be quite honest it sands like GARBAGE, as do all epoxy primers. And personally I do not even believe Dp primers are available anymore in the US for around 7 years. You can use the DPLF line that is basically the same minus the lead content hence the LF. They are NOT meant to be sanded period. They ball up, gum up, and sand like rubber.
3. My suggestion for a primer is two options. Use epoxy over your bare metal and filler work, but then follow directly with a SANDABLE primer surfacer. And at that point it is ok to get a surfacer that does not have corrosion resistance since you are using the epoxy under. Second option would be to just use a direct to metal primer surfacer. One great product from ppg is NCP270. Has wonderful fill properties, sands nice, and is very good for corrosion resistance.