LS,
The major reason that Britain (and others) transitioned so quickly in the latter 1700's to iron naval guns was because of cost, so when iron founding technology had developed to a point where the guns were deemed more dependable, the switch from bronze to iron was underway. Any reliable history that you read is going to relate how these guns were utilitarian in nature, they weren't meant to impress with their surface finish, a greater amount of time and money given to the manufacture of the gun's bores made more sense.
Even by the time of our Civil War, while the North had the manufacturing capabilities to machine even the largest Rodman exteriors to a smooth finish, the South didn't have that luxury, so it was decided by their leadership that Tredegar, Bellona, Selma, and other foundries were going to leave the exteriors of their cast iron Brooke rifles and smoothbores, Columbiads, siege mortars, etc., in a fairly rough state, and concentrate their energies on increasing the quantity produced.