BF is an awesome company that has more tricks up it sleeve than a bus load of jugglers, but powder coat is not the answer for rust. Most powder coat remains permeable. But given the fact that the problem is largely imaginary, powder coat is certainly a good product. It's main advantages though are film thickness, and the process.

For paint, irst you need to prep the frame so it is able to wet. This can last as little as a day or an hour so you need to be able to move on to the painting quickly. Then an epoxy primer, followed by paint would probably give the best results in a paint There are many interesting coatings used for other products like guns, but they are probably all too expensive. There is normally zero problem with a well maintained exterior. There will probably be some rust inside the frame. You can spray in there, but for the most part in normal conditions, and those described by the OP, the best practice (other than stainless) is to just get into the whole thing, become a user and learn to love steel the way it is, which may includ tiny rust marks long after you have given the frame unconditional love, and won't blow a gasket over a small blemish. I think in the right bad conditions, (you don't store your bike on a carpet, you do have real rain) Keeping tubes well oiled internally is the ticket.