I second the brass brush. That is my go-to method. Now, that does not work forever. Needle file sets do need to be replentished periodically, but they are not that expensive. I am lucky and have a Harbor Freight store not too far from me. They sell them.
Small files are my normal seam cleanup tool. I find they take off material faster, yet leave finer scratches, than sandpaper. I find a single coat of a good primer covers the fine scratches well enough.