Talk to the techs w/Brownells, they have many good pointers. I have done a couple over the original blue, worked acceptably well. Polishing will work better. I'm sure you know that Oxpho applies a phosphate type material, so in my opinion is better than most other cold blues. Make sure the metal is CLEAN, I use Mean Green, same as simple green without the scent. Warm the metal with a heat gun till mildly uncomfortable to touch, makes the chemical activate better. Apply with a wad of degreased 3/0 or 4/0 steel wool in even strokes till the color is even. I always wash the finished metal with clean water, dry with compressed air, and dip in ATF, then set aside to drain. Makes a dark blue that wears well. Give it a try, I think you'll like the result. Not as good as a proper acid blue, but a lot less work. Goatwhiskers the Elder
Edit: I do use the liquid, never tried the paste.