It may also depend on the coater! The place I use is the only place in North America that the Military will use because of their strict guidelines on the treating/cleaning processes. The Ecoat is PPG epoxy! I have seen panels ecoated by other companys that looked terrible! I personally wouldn't leave a primered car or part outside in the weather. We were told years ago by the PPG reps that epoxy IS porous and UV rays will break it down. It should be top coated if it is to be subjected to the elements of nature! Also any time you have two pieces of metal sandwiched together you are not going to get fluid between those panels unless there is a slight gap or seperation for air and fluid to flow. There will always be some form of rust between pinchwelds and laps unless they are unfolded or cut apart and cleaned or there is enough gap for fluid and air to flow!
A gray scotchbrite or equivelant is all that is needed to lightly scuff the surface for the topcoat or filler/glaze to "bite".
Brad

yep,it has alot to do who your e-coater and what they use, for simple example take acheap aftermarket fender or a hood from China and a OEM part,you barely scuff the cheap fender with brown scoth pad and you see bare metal,because the e-coat is very thin,but e-coat on OEM part can be sanded with 180 grit for a while before metal will show

I went with a slightly different rout with my bee. Since I had it balsted (and it rained directly after that) I needed some quick protection for the metal. I sprayed the Must for Rust on it. This nuetralizes existing rust and leaves a very thin coat of protectant on the metal. This then will be sprayed over with an epoxy primer and since it is water based it is non toxic and I can get it in all the tine crevices and it does not have to be neutralized. Good stuff. Here a shot of the application before drying.