>>6906519Rusting is really up to your taste, I really like those dark spots tho.
I remember reading that the Shermans were simply sprayed with green primer at the factory, which means it wouldn't chip but it could fade away, so you may want to take a look at that if you really feel like tryharding it.
I also found a guide, maybe it could be useful:
http://www.finescale.com/~/media/files/pdf/marketing/shermantank.pdf 