Testors acrylic thinner (green bottle) is the best I have found. Also, I recommend Iwata Medea. They are expensive, but you get what you pay for. I have been abusing mine for 2+ years now and it runs like a champ. YMMV.
As far as rusty stuff (back on topic), after you get the color red you want, gloss coat your model. No......seriously. Then mix rust pigment with burnt umber oil paint and a little bit of white spirits. (Thinner) Flick onto model with stiff brush. When sufficiently coated, put pure thinner into your airbrush and diffuse the rust spots. Not TOO much but you'll get the gist soon enough. After wards you can streak the oil and rust with a thinner dampened brush. Then dull coat your model when finished.
Do this in a ventilated area or you will kill braincells BTW. If you want any help in person, let me know and we can do a workshop up at WOW to get the technique you are looking for.
Hope this helps.