Firstly welcome to the ALR, the best place on the internet to learn about the American Longrifle.
It was Made in Pennsylvania, probable by Germans or Dutch, they had no care for how it looks on the outside, but inside it looks good.
I'm a little confused by this. Are you referring to your rusty Italian barrel?
Do you mean your new Colerain barrel that needs to draw filed to smooth to finish it on the outside?
Do you mean your rifle is poorly finished and that statement implies that originals by the Pennsylvanian Dutch were intentionally done that way?
Even the so called Barn guns of the 19th century were finished with some care.
Take a look at the museum to see how originals were really finished. Some of these show age and neglect but you can tell they were finished as new with a great measure of craftsmanship, especially considering the conditions in which they were built.
If you best is not good, it's still your best and the folks on here will help you improve on that. All you can ask of someone is to do their best.
On the other hand if you are not doing your best have no interest in improving and even make the very untrue excuse, original makers did no better then that's unfortunate.
Some of the "blacksmith" made guns may have been poorly finished, but even those guys did their best or were at least trying.
So do your best and the builders here will help you out as best they can. Make excuses for poor work and statements not based in fact.... not so much.
As far as your barrel problems....
Clean after each session, water need not be hot.
Run white patches till clean
Since Sperm oil is illegal, use a good quality gun oil
Re check, re clean and re oil after a few days
Seasoning barrels is baloney, get the bore clean and oil it
Most modern substitute powders are actually more corrosive than BP so clean religiously no matter what you use.