Another easy way to get a nice weathered look is with chalk pastels. It's been used in model building for a while.
First get your paint down, then a good matte coat. Next get a fine piece of sand paper to use as your palette and a nice assortment of chalk stick pastels and some cheap paintbrushes.(stiff bristles preferable)
Lay your sandpaper on a flat surface, and rub the pastel stick back and forth on the sandpaper, until you have a small pile of fine powder, then dab onto desired surface area with your brush. Go light at first, you can always add more. You can mix colors to get the desired look. Can be rubbed on in larger areas with a tissue, paper towel or cloth. Really an easy process and can get some great results. Practice on some scrap to get a feel for it first but easy to master.
This rc plane was weathered using pastels.
Hope this helps someone.