I’ve not used it, either, but since it works on plastic, I’d guess it to be thinned, water-based paint.
You can make your own for less: I’ve used Pollyscale (no longer produced) thinned with distilled water. Whatever brand of paint you choose, add a drop or two of liquid dish detergent to the thinned mixture. This cuts the surface tension of the water, allowing the paint to flow much more readily. Use a soft brush, at least 1/2" wide, and be generous with the application - if you haven’t used too much paint, the effect will be rather slight. This gives you control over the final appearance, as you need only to add successive coats after the previous one has dried. If you make the mix too paint-heavy, it’s almost impossible to undo.
Other methods and materials for weathering things include airbrushed paint (water- or lacquer-based), dry brushing, and chalks or pastels.
For airbrushed weathering, I like to use no more than 10% paint, with whatever thinner is appropriate. This, like well-thinned washes, allows you to build-up the weathering until it matches what you want. The trick is to know when to stop. [swg]
Dry brushing can use any type of matte paint. Simply dip the brush (sized to suit the effect which you’re trying to achieve), and wipe as much paint from it, back into the bottle, as you can. Next, wipe the brush on a paper towel to remove even more paint, then use what remains to create highlights or shadow effects where you want them.
Using artists’ pastels for weathering is a good method if you’re just starting out, as it is reversible. I generally buy earth tones, along with black, white, and grey. This will allow you to create pretty-much any colour of dirt that might be appropriate on rolling stock, locomotives, structures, and even scenery (the last one is not always reversible).
To apply pastels, I like to rub the sticks on some coarse sandpa