That is interesting. I also found that most of the "discoloritis" was clay, dirt, grease, etc. Lots of white paint under there. Took degreaser, scrubby sponge, sandpaper and a pressure washer to get that **** off. Lots of elbow grease from my 11 year old as well!
Outside of wheels was less rusty than the inside.
I agree with Jimmy that just starting with the front wheels is probably the only way to reliably get them done!