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I like using real rust. I make mine by letting a piece of super fine steal wool rust and then grinding it up as fine as I can. A little flat clear spray paint holds it in place.
Artist oil colors work good too. They allow a lot of working time and blending.
Lots of videos on youtube about weathering freight cars. Many of them use artists oils for rusty streaks. Just type in Weathering a Boxcar into the search box.
You can get a lot of ideas from this site for weathering. You don't have to take it as far as those guys do, but the practices and materials would apply to what you want to achieve and echo's what's been said here already.
Thanks for the link!! There's something I can work with....
Makes me want to make a piece of rolling stock look like its past retirement, and ready for scrap!!!
I do agree with that!! I have a UP mainline with a siding 1/2 mile from my house! I constantly get to see rolling stock in many degrees of weathering, parked!