Liquid gun blue, by Birchwood Casey for quick touch-up of barrel scratches or worn spots. Perma-Blue® produces a blue black finish on steel, except stainless alloys. Always prepare the surface with degreaser before using this product. Easy, but requires repeated coats. Paste gun blue works more slowly, but many users prefer the easy application and controlled action, allowing touch-ups to blend perfectly. Apply it to a cleaned surface, wait 60 seconds, and wipe. Repeat as necessary to achieve the depth of blue desired.