Trigger is metal and has a rust/corrosion resistant (not rust/corrosion proof) finish on it. The frame is polymer and doesn't rust.
To take metal off the bottom of the trigger, even if you had a way to refinish it effectively, you might have to install it, try it, pull it back out, remove some more metal, install it, try it, etc. By scraping/filing/sanding the inside of the trigger guard can be done with the pistol assembled and you can test trigger clearance as you go so you only remove enough to make it work.
I didn't even try to make mine look nice. Scraped it with my pocket knife. Either it's in a holster (where you can't see the trigger guard) or it's in my hand (no one will be trying to look inside the bottom of the trigger guard) or it's hidden/out of sight where I keep it when not wearing it (and no one can see the inside of the trigger guard, or I'm shooting it (and it's so much fun to shoot a pistol that shoots nice groups I don't even think about the trigger guard.) Actually, the .40 S&W P09 doesn't really impress me that much (group size) as my .40 S&W P07 DUTY and my 9MM P09 shoot better groups. But my wife used to think it shot really good groups and I thought I was going to "lose it" before I got her that last CZ75 Compact. She's happy with the small groups that gun shoots, so I got to keep the P09 .40 S&W. Funny how that works.