I have used the link tutorial,in addition good old chalk pastels from any art shop.Rub them on with finger,use sandpaper on them to create a powder and sprinkle as much or as little as you like.This then blends the colours of the base weathering and does not need fixing with hair spray.
For scratch marks use rubber helmet trim ,cut a one inch piece and strick it on your armour,leaves a nice black mark on edges or high spots and does not scratch because its rubber.A nice Subtle touch.