I'd like to sand some wood planks on a rustic table to remove paint stains, but keep the rough wood structure (1-2 mm height difference). How can I do that?
Use sandpaper and your finger, or the edge/corner of a sanding block.
Though if you're just trying to remove paint I might first try using acetone or other solvent.
Use a chemical stripper to remove the paint and a very fine grit sandpaper. You can get it clean this way without sanding away any wood.
You can use a wire brush. You would want one that is pretty stiff.
Whatever approach you decide - test on the bottom of the table first.