take a dremel with a pointy reamer bit in it and the start gently tapping the sides of your car where you want the rust holes to be. prefereably do this on a really fast setting hot enough to melt the plastic. when you have some nice holes hat are about the size of a scale penny (about the size of a pen dot), turn the dremels speed way down and then start cleaning away the globs of melted plastic from your holes. then paint these holes a and the general area around them. also paint any place on the car you want to be rusting through this same rust color. then take a masker fluid and cover up the holes and wherever you want there to be rust showing through. make sure you have the rust colored paint under the masker! then sprasay your normal paint job with plenty of paint to build up a little bit of paint thickness. this will simulate the the fact that the rust, just like on a real car, is under the paint. then remove the masking fluid and you will have rust spots. take some rust colored pigments and, using a brush streak down from the rust holes. hope you liked my essay on weathering lol. if you really want some good tips, go over to armorama.com and ask the resident rusty tank experts over there. lol