removing scratches

Just wondering how can you remove those fine scratches after filling and sanding the model.

Use progressively finer grade sandpaper (wet-n-dry). I usually start with 320 grit, then move on to 400, followed by 600. This is normally good enough. You can find even finer grit (1500 or 2000) if you need it extremely smooth. For natural metal finishes, after the finer grades, I polish with toothpaste. Good luck.