So, what you have there is most likely a late 18th century or early 19th Century firearm known as a dragon, a type of blunderbuss particularly favored for maritime use. From the photos, that appears to be a brass barrel, correct? If so, that further suggests a naval or maritime origin. Do you know whether it's original or an aged reproduction?
What's the plan? I would discourage much in the way of use. A brass barrel of unknown age is not something I'd be willing to light off 3 drams or so of black powder in...
There does appear to be some light surface rust on the lock work where the protective lacquer has worn away or was not applied. Some judicious use of steel wool followed by Barricade, for example, should solve that. Other pieces of the lock work seem quite rusty under the lacquer. And the brass barrel seems to have been hit with the lacquer too. Getting that off isn't too hard, but, again, what's the plan? If it's to be a wall hanger, stopping further rust on the non-lacquered bits would be a good place to stop, or that and clean off the brass. If we knew it were a genuine late 18th early 19th Century piece, I would say stabilize the rust and leave it at that. If it's some kind of reproduction, by all means, go to town with the methylated spirits and phosphoric acid, plum brown the lock work and clean off the brass.