OK. I guess that I'll chime in here although I'm sure that the others here are just as reputable as I am concerning paint, and what techniques that they use, and why. I'll keep this short and sweet - to the point.
I'm of the REAL OLD school ideal of doing it accurately if it's something that your trying to replicate. Do not deviate, or use artistic license unless you're doing your own thing. I also use a simple rule of thumb regarding modeling.
Do not do what's popular such as using what others suggest simply because they've read it somewhere, or because they know someone who did it, and got lucky with the results. I understand that not everyone here has the same level of experience. However there's no one way of doing things to achieve the same effect since there are so many mediums to use at our disposal these days. I guess that it comes down to a matter of what works for you, or rather what you can do with what you have on hand, and what you can do with it. I didn't read everyones' repsonse here just to keep from colouring, or infulencing my own opinion, or thoughts on this subject.
I don't use pen (ink) washes, or oil paints, or art acrylics, but that's because I use automotive paints, and enamels that are to be used on wood, metal AND plastic. I know how to weather without using chalk. I use painting techniques that are simple, and easy once you figure out how to do them. This is what we did back in the day, and it's what ILM, Apogee, and other studios before, and after them did before anyone thought of using ground up chalk sprayed with a clear coat. Plus the old school way of doing things was easier, and more durable with paint. No messy ink washes, or water based paints to ruin our models with.
Sandpaper, steel wool, baking soda, drywall mud, earthen clay, superglue, and papier mache were our tools of the trade for the most part. Regular house paint brushes, and old rags were what we used to weather vehicles with. Thinned down paint with various shades of other paint were applied to resemble weather effects of rain, dirt, rust, etc. to walls of buildings, houses, and machinery to look old although it was all new. It was hard to walk onto a set, and realise that a lot of this was FAKE!
Look at others, and what they use, and decide for yourself what you like, and how to do it. ME - I'm a purist, except for cars. I paint them the way I like them. Not neccesarily the same as the model box pictures.
~ Cobra Chris