Sometimes there is sand embedded in the surface of the castings, which wreaks havoc on the files.
Start with coarse, and work your way to fine files. For traditional finishing methods, you can switch to scrapers, the same ones you use on wood, and then to burnishing. For contemporary, you can use wet 'n dry sandpaper, then a buffing wheel. And yes, separate files for brass is a good idea. I may think I can't afford an extra set, but having spent HOURS filing brass with a semi dull file, lemme tell you, my argument for economy rapidly loses steam.
Many originals show file/scraper marks inside the bow. Contemporary pieces often show a high level of finish. So how you finish your brass is entirely up to you. Consider how you will finish the wood, and have both materials finished to comparable levels.
Happy New Year
Tom