I sanded those plates with machine before bending to 120-150-180-240-320-400. After bending i sanded with my hands to 400-500-600-800-1000-1200 and finally polished with autoclym metal polisher. It's not actually hard work, but it takes time. Mainthing is to use a lot of water, if the paper is too dry it's get stucked and it makes visible cratches. If you sand 400 grit to vertically, you must sand 500 grit horizontally.. etc. You can see the colour of the plates changing when you are sanded enough. Before changing paper, every scratch has to be sanded down. Those final, 1000 and 1200 i sanded under the shower, it makes better surface. Mainthing is that there is no shortcuts, you can't jump 400 to 1200, every scratch that 400 paper left is still visible. With alumine needs to go to 1500 or higher, but you can make bigger "jumps" 400 to 600-800-1200-1500 grit, and finally maybe some sandingpaste, then polishing. (if you don't wanna see any scratches And, thanks for comments.