Actually Albert, you can claim accuracy with this build.
I'm guessing you didn't come up with this idea all on your own, (even the title 'Hillbilly Guntruck' is what they are actually referred to in use) not only are there plenty of photos of this type of thing in Iraq/Afghanistan but I'm fairly certain Italeri based their box art on this exact vehicle.
Either way, research trumps opinion and it's just a few mouse clicks away where we can find plenty of photos of rusty steel up armor on painted vehicles.
It has nothing to do with rust not showing on camo, it's all about bare steel being added to vehicles as additional armor. Of course a bare steel surface is going to weather differently than painted surface.