Preparation is Everything.
Bluing steel should never be considered a method for "covering up" imperfections in steel. Often times people will ask us if they can use Van's Gun Blue to "fill in" scratches on a gun or cover up a stain on a piece of steel. The fact is, bluing will often times highlight scratches and stains on the steel surface. This is because gun bluing brings steel to a high luster which reflects light much more than a flat finish. Scratches and stains will reflect light differently which sometimes makes them more obvious. For many applications these differences create awesome effects and are very desirable. Examples would be bluing large plate steel for table tops, wall panels and rustic fireplaces or giving a vintage look to stair treads and banisters. On the other hand, if we are touching up or totally re-bluing a firearm, we are usually aiming for that perfectly uniform factory finish.
For all these reasons we always recommend thorough steel preparation. This starts with making sure the project looks and feels exactly how we want it before the bluing process begins. Getting the steel in "finished condition" is a matter of personal preference but we will highlight some of the more popular techniques.
Prepping steel with the least aggressive method necessary is usually best. Example: you wouldn't want to use #500 grit sandpaper to remove an imperfection if #1000 grit would have done the job. Starting to aggressively will just create more scratches to smooth out before bluing.
For this process, starting with the least abrasive method possible and working toward courser methods if necessary is always best. Steel wool and very fine sandpaper (600 - 3000 grit) are often preferred. Chemical methods for stripping steel can also be used. Pink Naval Jelly or soaking in simple white vinegar can be very effective methods of cleaning steel as well.
Once your project has the desired finish and appearance you are ready to move on to the next step: De-greasing.