Incomplete or just plain poor carding of the surface is a source of a less than stellar rust blue finish.
Not getting the red rust to completely convert to a blue/black coating can be the problem.
Then all the carding in the world won't lead to a nice 'blue'.
Lots of things figure in the latter and even when everything goes right, the blue isn't going to be of any higher luster (shiny) than the polish underneath it.,,with rust blue usually less.
It is possible to do high gloss blue w/rust bluing, it does take some extra care and time.
Some rusting soln's just naturaly etch the surface more than others, they are more aggressive. Some steels react more than others.
The carding process itself is a matting finish to metal wether it has bluing on it or it's in the white. Wheel speed, wire dia and stiffness,pressure applied all play a role.
The rust bluing process itself is no deep secret. Getting a repeatable result and being able to alter the result to your wishes to match for restoration purposes is the hard part.
