The same thought occurred to me concerning armour plate Gina. I've never inspected it closely but I wouldn't expect it to be smooth like sheetmetal, and that will make it impossible to isolate the factory paint layers cleanly, because they're only a few microns thick. It may help to rub back very small areas and take extreme close ups for analysis on screen. This can be a useful method for revealing very thin paint layers, as seen below. Weathering is a problem too of course. The FGT door has several layers of protective paint, including full gloss DBG, and it was removed from the vehicle and stored in a container for decades. It's not often we get such a good sample of wartime colours to work with, esp. Light Stone.
Darryl, your technique is fine, although 400 is a bit coarse as you say. I use 800 grade myself, with very gentle pressure, more like polishing than rubbing. Always wet of course, and pausing frequently to wipe off and inspect. However I suspect you've done as well as possible with armour surface. I'm puzzled by the photo, as I don't see any primer coat. Looks like green directly onto metal, with rust discoloration as a result. I don't recognize the brownish colour - is it wartime paint, and is the vehicle single colour or camouflaged? More pics needed, preferably in natural light if possible, and photographed wet for colour rendition. Also do you have any history on the vehicle?