When carding, you want to do it often. If you wait too long between efforts, then the rust can build up too thickly. If you do it often, then you can do it with a fine abrasive (like fine sandpaper or steel wool). If you wait too long, then you end up (unfortunately) having to scrape it off with courser material (or even a steel brush), and that can leave scratches.
Scratches can be polished out, but if you end up with them, it usually requires that you polish the entire part. It usually doesn't work to just polish out a section (like a section of a plate) and then try to refinish that section. It ends up splotchy. The part doesn't have to go all the way back down to bare metal, necessarily, but the amount of residue finish does need to be fairly consistent over the surface before you start browning again.
Did anyone mention that you can use varnish to protect threads before you brown? This is really helpful if you are using any sort of immersion. Just carefully paint the threads, let it dry completely, then gently sand off any varnish that ran onto areas that need browning. You can cut the varnish out of the threads easily with a tap after everything is finished.
Also, if you want an aged look, leaving it a little uneven is fine. Esp on early rifles, it doesn’t have to be perfectly smooth and even like a chocolate bar. There is good evidence to indicate that a lot of early rifles were either left in the white or were blued by the smith. The “browning” that we see on some of them is just natural rust that accumulated over time. So the amount of that rusty finish we add on to a modern reproduction will either represent a period piece as it was being used by a pioneer, with a light, uneven browning that accumulates during use. (Best if you are into reenactment.) Or it will represent a piece that was badly abused and or left in an attic for 100 years and accumulated a lot of rust. (Can look cool on a wall-hanger, provided the stock is also equally aged.)