Thanks guys! That rust is meant to be very subtle, I don't want the Empire's machinery to be in poor upkeep! That blast effect is part of the Jango Fett from AOTC Saga figs, and the explosive part, I can't remember where that came from! But they were combined with super glue and then the peg was filed down to fit into the front of the cannon.
As for the mud, that is something I've been kicking around as well. I'll have to try to give it a light coat of it on the underside just to see how it goes.