you can easily sand down stem quills and seatposts with a 200 grade sandpaper. you may need to deanodize the component first using oven off or a similar product with lye.

i did this recently for a seatpost, and it worked just fine. i'll use it in the frame, but i went a little too far with the sanding. the problem was, i used a low grade paper and kept trying the post in the frame, moving back and forth between sanding and fitting until it fit snuggly. but then i needed to use a higher grade paper to remove the scratches from the rough stuff. by the time i finished sanding the scracthes out of the post, i had taken even more diameter off, and it slipped in the frame a little too well. so i should have stopped with the rough sanding much earlier. it works, but i would prefer a more snug fit.