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As for the scabbing, I do that (this only works with enamel paints) and I recommend great care DO NOT GET THIS ANYWHERE NEAR BODY WORK PAINT JOBS I get a tiny bit of cellulose thinners on the brush a dab where I want the scabbing, do this very sparingly and build it up gradually allow the paint to set then if you want deeper scabbing go over it again you can do this upto 4 times after which I've found the paint will start showing the plastic colour underneath (THIS METHOD WILL EAT INTO THE PLASTIC) so you have been warned. I use dirty thinners as it gives that staining round the scabbing you see on rusty things, I have in the past (not on this as I don't want it that rusty) put dabs of pva before I paint the item which gives a raised part to begin with and if you want really heavily rusted you can add more dabs of pva over the first one but make sure it's smaller and irregular.
