Inside is the most important. That is where wet mud and sand will sit for years and rot it from the inside. You can always clean and repaint the outside. We just finished putting six feet of 5" x 3/16" plate on the CJ's frame to fix the rust holes and cracks. If I could remove all of the rust slabs from inside the frame they would probably weigh 10 or 15 pounds. We found the bad spots a spot at a time. Had we starting beating on it with a welding pick/hammer and found all the bad spots all at once, I would probably have gone with a new frame.
One guy taped off all of the holes, poured Por-15 or Chassis Saver inside and rotated and tilted the frame until every inch inside was covered, then drained the excess. That is what I would do. Maybe twice or three times. Now if you live in Phoenix, that is probably overkill. On the other hand, that inside spray idea using one of those killer coatings might be easier.
I just painted the frame (outside only) and a half quart of Chassis Saver gave me one coat, brushing it on. $40/qt at NAPA.
'84 CJ7, '07 5.3 LMG Chevy, D300, 4L60e, 33's, 4.10, rear Detroit Locker, FG Tub, HOK Tangelo Pearl