washes

You definitely don't use an Airbrush for any kind of wash...

Like Boba said, a "normal" wash is thinner than a pinwash and you use a brush to paint it on the whole model, let it dry a bit and wipe it off again, so it will give all surfaces a bit of "tonal variation" and stay in recesses, such as panel-lines. You can use different materials and mixtures there, such as highly thinned oil paints or Pigments/Pastels dissolved in Turpenoid.
It also depends on your finish, how well the wash will stick to your model. If you only want to accentuate your panel-lines, you want to give your model a high-gloss finish (such as Future), so the wash will be easy to completely wipe off and only stay in recesses (for wiping it off, you can generally use the same medium, that you used for thinning, i.E. Turpenoid, but not too much, else you'll wash it out of your panel-lines too). That's, what you would normally use on an airplane. When you're satisfied, you can seal it in with another clear coat (matte or satin), to give your model the finish, you want.

To use a wash, to give your model an overall dirtier/varied look - as usually seen on Tanks - you can apply the wash over a matte surface. The matte surface will make the wash stick everywhere and it will be something between hard and impossible to totally remove.

A generally easy-to-handle and beginner-firendly product for that are the Promodeller Washes (http://www.promodeller.com/). They're clay-based and can be rubbed off easily.

Pin-washes are a bit thicker (like strong coffee) and you use a very thin brush (you could also use a pin, hence the name) to lightly touch a panel-line and the wash will flow from your brush and along the panel-line followingcapillary action. This will also work on the base of raised detail like rivets. It's a more selective method and you can use it to put in more accents, rust or whatever... You can also use combinations of both techniques...

hth :)

Boba - thanks for the tips. I still am unsure about a "pin" wash? Is that a way of saying using a fine brush instead of an air brush.....or do you ever use an air brush for washes?

Also, after applying the thin paint, do you allow it to dry or blot it up?

Honestly, you have to experiment. what a pin/dark wash is is a thinned mix of dark paint that's flowed into recesses. These highlight detail and give dpth to the finish. The type of paint you use depends on the wash's paint type. If you paint with enamels, use acrylic washes. If acrylic, use washes with oil paints. Take a dark color, (Not quite black, a dark brown) and thin it so it's the consistancy of strong black coffee. Use a brush and tuch recessed details like weld lines, and raised ones like both heads. An overall wash is even thinner, but is applied to the whole model. be warned, the wash WILL darken your finish. I would only use overall washes on armor, and sue pinwashes on AC panel lines and such. Hope it helps!