The Birchwood Casey Perma blue paste is an excellent way to touch up scratches, worn spots or even completely re-blue most guns and prevent rust.
It will give a non-streaky, even blue-black finish to steel (except stainless).
Perma blue paste gun blue provide great bluing to metal that has already been thoroughly cleaned and is free of rust.
Perma Blue Liquid Gun Blue will not blue stainless steel, aluminium or non-ferrous metals.