Play-doh, I agree with the above posts, however, if you choose to use pastels/chalks, you will find the lighter colored chalks will dissapear or lighten considerably when sprayed with a protective coat. There are quite a few techniques-all have their own pros and cons, but I find if you do some washes first on the model and then apply the chalks, there tends to be enough "tooth" for the chalks to stand a reasonable amount of handling. I have cars that have seen regular use for several years with no noticiable loss of chalk. One advantage is that after a few years or so, just add a bit more chalk if necessary, I personally haven't had to touch mine up at all. The important thing is that the car has a certain amount of tooth to the finish to retain the powders. This is just one viewpoint on this subject matter, and there are more than one way to skin a cat, but I have had good success using this method. Also, there is a company called "Bragdon Enterprises" who manufacture powdered pigments with a binder added into the formula-this helps the powders stick to the surface. They are a bit pricey, but I have seen some nice results with them. I personally prefer artist quality dry pastel sticks from an art supply store. These will last you forever with many more colors to choose from-plus, they are forgiving if you initially do not like the results. Remember, the washes will provide the tooth if applied properly. There are many viewpoints on this matter,and many work very well, but this is a nice way to get started on car weathering. Hope this helps.
Stuart