Physico-chemical-weathering

Chemical stage 1: Base coat, e.g. dark green from Tamiya.
Use colors with acrylic resins like Tamiya and Zero Paint.
Make tests of the described procedure prior to paint your model!

Chemical stage 2: Apply a thick coat of hairspray
and let it stay for 30 secs. Then rise with warm water.
The alcohols and solvents in the hairspray will
chemically alter the base coat. See change of color between
the upper 2 pictures!

Physical stage 1+n: Carefully treat the surface with
tools like wooden chisels. Be careful not to score comletely
through the paint work when using metal tools!

Rub, score, scratch and strike the surface till it looks worn and used. Apply chipping with songes, straight scratches with a curved blade dipped into thin paint. Apply panel washes before applying the hairspray. Add sprinkels and more washes as desired. Add streaking effects by airbrush using heavily diluted Tamiya flat black and redbrown.