Pastels, not the roundtrees ones
I bought a set of earthtone soft chalk type pastels from The Works.
Don't get the oil pastels, I made that mistake a few years ago.
Get your chosen pastel, rub it on a piece of sandpaper, only a little.
Then pick up the dust with a non synthetic brush. I find squirrel to be the best, nylon is too smooth and doesn't grab the dust.
Then just paint it on and you'l get an airbrushed look without the hassle of expense. If you make a mistake, just wipe off with a damp cloth. The important part is to "FIX" it with a spray of matt varnish.
Just spray a margarine tub and have a practise, you'll be surprised.
It's a lot softer finish than painted weathering, although the base coat does get a burnt umber wash first to tone it down.
Hope that helps.