Kevin, Jim gave you a great way to weather.....with chalks. If you don't like theway it looks wipe it off with a damp rag.
Some of the tools I use to apply chalks are cheap and easily found. Paint brushes, I have a set just for chalks, the wife's eye shadow applicator, just don't put it back when you are done or her eyelids may look weathered(Wal-Mart has them on the cheap), and cotton bals, you can buy a whole bag for a buck at Dollar Tree.
As for the chalk, I bought a artist's thing at Wal-Mart for about $8, I think. I also use: 1.) Ashes, dirt(yes, real dirt),
2.) Baby powder(it leaves my cars feeling and smelling fresh and prevents diaper rash, just don't use your wife's $50 body powder. She may get upset with that.) Flour also works.
3.) Charcoal dust(works really well for "griming" a car or engine),
4.) Pulverized limestone(right out of the driveway).
5.) Real rust. I find an old rusty pipe and sand off the rust onto a sheet of paper and place it in a pill bottle. An aspirin bottle usually does the trick.
I probably use more of the "natural: things I find more than I use actual chalks. And MUCH cheaper.
After you get a look you like, spray with dullcoat.
I started weathering by doing the trucks and such on my cars and engines. I grabbed an old car and engine body and started practicing. That's what is so neat about chalks, you just wash off the bodies and try something new. You have a new "canvas". Start small and graduate to larger, more complicated, things as you gain confidence.
Bob