Proxon pen sander works well. You can find the tool handle bit for ~$40 online (eBay). Then just cut the weird plug and wire it up to a 12v DC barrel jack and use any adaptor... Or pay $90 for one with their power supply.

You will be stuck buying their sandpaper. It is high quality, with an extraordinarily good adhesive. That is the secret that makes this work. I am a former pro auto body painter. I've used tons of sandpaper (not hyperbole) and I've never seen another with this type of adhesive.

BTW, never use a rotary tool for finishing work on anything. This will dig into and make uneven nonsense. Pro auto body work is 99.9% sanding and prep work. It is a multi billion dollar industry. There are no shortcuts here. There are the right tools for the job, but no shortcuts.