There is "aging".
It is based on the idea of creating the condition of a well used poorly taken care of gun with most or all of the finish worn off of the barrel and lock.
Several different methods have been used varying from painting the de-oiled barrel with yellow mustard or painting it with vinegar and letting it sit for a day before washing off the stuff and reapplying it.
Some have taken to wrapping string around the barrel in a spiral pattern and soaking the whole thing with vinegar. That sometimes produces a spiral pattern on the barrel.
Navel Jelly, commonly used to remove rust will etch the surface of the barrel if it is left on for a long period of time. This produces a soft dull gray metal look that some like.
Distressing the surface by applying a chlorine bleach for a while will produce severe rust and pits making the barrel "age" 100 years in about 15-30 minutes. The loose rust is wiped off leaving a rusty mottled pitted surface.
Some have combined these with an application of cold instant bluing like Perma Blue. After applying the quick bluing agent the bluing is rubbed off in the areas where a hunters hand would have grabbed the gun to carry it. This is sometimes followed by mustard or other rusting agents. Navel Jelly will strip the cold bluing (or any other type of bluing) so most folks do not use it for treating a instant blued surface.
There's the Polished surface which is buffed to a mirror like finish. Surprisingly, a mirror bright finish is quite rust resistant if it is waxed.
That's just a few of the things that come to mind at the moment. I'm sure you will hear of others.