Birchwood Presto Mag

Birchwood Presto Mag is a cold blue liquid gun blue for bluing all types of steel except stainless steel. It is used for restoring and re-bluing firearms to provide a deep blue-black finish. This product is suitable for both full re-bluing and touching up wear spots. It is a quick-acting and durable formula that does not require heat for application.
Key features and uses
Product type: Cold blue liquid for firearms.
Primary use: To restore and apply a new finish to steel gun parts. It is designed for complete reblue applications and for making damaged steel parts look new again.
Finish: Provides a deep blue-black luster and a durable finish that will not rub off.
Compatibility: Works on all types of steel, except for stainless steel.
Application: It is a liquid that is applied without heat and works quickly.
Preparation: For best results, the metal surface must be thoroughly cleaned and degreased before application.
How it compares to the Presto Gun Blue Pen
While the liquid Presto Mag is for larger re-blueing jobs, the Birchwood Casey Presto Gun Blue Pen is a similar product designed specifically for quick touch-ups on scratches and wear spots, as explained in this Amazon listing.
The pen is easier to use for small areas because it uses a no-drip, fine-tip applicator.