By far, the number one reason a person will have trouble bluing a piece of ferrous metal is the presence of oil. Grease, dirt and oils prevent a direct connection between the bluing and the steel.
A couple times a year we will get a call or email from a customer basically explaining that they just completed a successful bluing project and they didn't even clean the steel first. "That Van's blued right through the dirt and grease." they'll say. Others have suggested that Van's itself be sold as a degreaser... While this might be a nice way for us to sell more cold blue, it would certainly be more expensive and in our opinion less effective than other methods available. While we appreciate these testimonials, they are rare and we never recommend trying to blue unclean surfaces.
In almost all cases oils will block uniform penetration of the bluing resulting in a finished project which is uneven and spotty. Dirt, grease and oils should be completely removed before bluing. Degrease using the methods and materials you believe will work best for the steel you are bluing. Acetone, denatured alcohol, brake parts cleaner and even heavy duty citrus degreasers are all effective chemicals for removing both synthetic and natural oils. Sometimes the build-up and deposits on a project are heavy and complex (like an engine block) and require a combination of cleaners but most often any one of the above mentioned degreasers will do the trick.
Often we think that if we sand, steel wool or even bead blast a piece of steel that we must also be scrubbing off any dirt and grease as well. Be reminded that all of these products contain resins and oils themselves which will leave behind a residue that can block the bluing process.
Bottom Line: Once a project is degreased nothing should physically touch the steel except Van's Gun Blue! Even fingerprints can leave a mark but since we always wear protective gloves when working with chemicals this shouldn't be an issue.