It depends what sort of effect you want. Quite oiften the Germans would just slap it on with what ever was to hand, paint brush, rags, mop. So it doesn't have to be all neat and tidy.
If you want a worn effect, apply hairspray and then once dry the white wash, must be acrylic. Once that is dry take an old and some water and remove any paint you wnat.
But eitehr way check referance pics, you wilkl see just how slapdash it was applied.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3