IMO, polishing does have a definite effect if we are talking about browning steel or metal resisting rust.
If you plan on doing any rust browning using one of the solutions like Laurel Mountain, it works best if the barrel finish is not smoother than a 320 grit sandpaper would leave.
If the metal is finished with a 400 or finer grit or polished before applying the solution it will only "bite" into the metal in some areas and not at all in others resulting in a very mottled finish.
These true rust browns usually roughen the surface so even if one can apply enough coats to get a full coverage, much of the polishing will be destroyed.
Birchwood Casey's Plum Brown on the other hand doesn't care how polished the metal is so long as it is free of all oils (including your fingerprints). Plum Brown only lightly coats the surface so a polished steel surface will become a polished brown surface.
Before you jump towards the Plum Brown, it has its problems too.
While it works fine on smaller parts like a lock plate or a steel trigger guard or butt plate it is difficult to get an even, uniform color without applying at least 4 coats.
It is a hot process where the metal must be heated up to 250-300 degrees F before it can be applied.
If the temperature is less than 250 degrees F. it will form copper plating that must be sanded off before any browning will happen.
If the temperature is greater than 300 degrees F. it will splatter, bubble and dry leaving brown spots and crusts that must be washed off before continuing.
It also gives off some really bad smelling fumes while it boils off which from the smell of them can't be good for a persons body.
As for the polished brown surface Plum Brown can leave, to most gun enthusiasts it looks rather phony. Its smooth brown appearance isn't something one would see on a original gun in very good condition and it definitely isn't something that would naturally happen on a old Antiqued gun.
On the other hand, if the metal surface was left rather rough, Plum Brown can look quite good (if enough coats are applied).
Getting back to "polished" steel, if a barrel is sanded to lets say, a 220 grit condition and left alone it will begin to develop light corrosion that will eventually turn into hard brown rust. This might take 100 years but it will happen.
If the metal surface is highly polished it will pretty much remain a bright shiny surface for years if no one handles it. (In addition to oils, bare skin will leave a mild acid that will attack bare steel, leaving dark fingerprints. Waxing can keep this from happening.).
