Thanks!
I will just copy the text I wrote on another forum (if you do not mind) to add to the YouTube video):
It is a technique meant to depict paint jobs or or whitewashes that are wearing off, leaving the underlying scheme or base coat to show through. Essentially, you paint your model in the colour you want to show through in the end and then give it a coat of gloss varnish (to create a smooth surface to work on). Then you apply the hairspray and once dry (it is best to leave a couple of hours between he steps) airbrush the colour you want chipped off to over the hairspray. It is important to use an airbrush, since you need a virtually dry application. If you brush it on, the paint and hairspray will mix and the next step will not work anymore. Afterwards you use a brush with stiff bristles which you dabbed in warm water (best to remove some water from the brush so it is not completely soaked) to rub over the model where you want the paint to rub off. Once that is done, you can do the normal painting, weathering detailing an so on.