First off, hello to all, this will be my first post
This is how I create rust
Purchase a box of SOS pads, (the ones without the soap). Get 1 or 2 of them wet and put them in on old soup can. Let them sit for several days to a week (I never timed mine) Eventually most of the pads will turn to a rust powder.
Grab your model, and on a pallet,, mix up some rust and flat black. I first paint the model all over with the rust color, then go over it with a mixture of the rust and black for darker areas.
While the paint is wet sprinkle on some of the rust powder, making sure not to have any of the un-rusted particles in with it.
I had to go over mine a couple of times in some areas to get it just right, even painting over some areas to tone some down.
For larger areas or clumped rust you can dab on some CA, and apply some baking soda, then go over it with the above process.
And the cool thing... it's real rust.
Glen
The areas rusted through is done by carefully grinding the inside of the model with a dremel and a cutting bit. Then poking it through with an xacto blade.