I tried the salt technique, as well as using Humbrol Maskol.
The salt technique was fine up to a point, but reacted with one of my paint mixes. All the other mixes were fine, so I'm assuming it just needs a little fine tuning.
The liquid mask method, in my case Maskol, works OK, but does not look as good as the salt technique.
However if you want that generally scuffed and faded look, I use a fibre glass pencil, and just rub the surface gently. This can be done sufficiently to take you back to the base coat, or just enough to suggest a scuffed area of paintwork.
Karl