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how do i get that surface smooth rust.
There is a product called Rust-All that works pretty well as well as as set of rust powders that come with a binding adhesive mixed in the power. They also work well (can't remember the name, but I got them in a railway model shop).
I must reiterate, though that the sort of rust you are seeing in the photo just wouldn't be present on a real vehicle during the war. All of these nice looking models with paint chips and rust stains, well they look nice, but they are generally not accurate. Factory applied paint just does not chip that way and American vehicles especially, had really good paint. A bit of fading, sure. Lots of dust changing the tone of the pain? Yep. But wholesale paint loss? No, except right on high contact areas and these would show as burnished metal and not heavy rust..
Now, if you are doing a vehicle tha has been sitting around for a number of years, or a halftrack that has been converted into a farm tractor or the like, then sure, recreate the weathering as you see it in the photo. But anything in use during the war just isn't going to look like that photo, no matter how hard it was used.
My 2 cents
Paul