A little rust was hiding behind the belt line/outer glass sweep. The top panel just below the glass is made of the outer panel and a smaller inside panel, bent to the inside and spot welded. Am I the only one bothered by even a tiny bit of hidden rust? The best way to rust proof is to have the door upside down and let the rustproofing oil run into the small gap between the outer and inner panels. Using a thin spray nozzle I tried spraying upwards into the gap and watch for the oil to seep from between the sections.
While you have access to the door's guts take a look at the filler at the door reinforcement to panel gap. During assembly globs of glue like material is placed between the reinforcement and outer panel to prevent movement and vibration(I guess). Over time the glue filler falls out. Replace it with blobs of silicone at the top and bottom of the bar.