The coin door skin is stainless steel with a “grain direction”. I say that as the SS can be polished to a mirror finish if you care to do so.

The bracket appears to be plated steel and has no inherent grain direction.

The only way to get the scratches out is to sand it down to below where the scratch actually is.

The only way to get the grain out (which are just uniform scratches from sanding) is to basically work your way down to around 1000 or 1500 grit and/or use a polishing wheel (rather than hand polishing). At that point, you can usually get a mirror finish when polishing.

Wet sanding also helps.

I also like to use abrasive balls from harbor freight to knock off any surface imperfections and scratches before going to finer grits to prepare the surface for polishing. Although, when I've been in a hurry, I've just used those abrasive balls and polishing wheel and ended up with decent results.

No.

You need more sanding to get below the pitting in the metal from the rust.

Just FYI, PBResource may still have nee coin door faces available.

I suspect the original coin doors aren't plated.

I just noticed the right side is crumpled from prying.

I'd probably cut my losses and just get a new door. I wouldn't waste my time trying to refurbish a door in that condition if I had another option.

The door is stainless steel. It should have an even grain running horizontally. Sand with a lower grit to get the scratches out. Don’t go above 220 grit or you will lose the grain. Wet sanding is best. It’s important to sand evenly and exactly horizontally. Use a piece of wood or whatever to sand against. Your
hand will never sand straight horizontally. Finish with a red pad(like scotch bright). Unless you want shine/no grain, then go higher on grit and buff. You can go all the way to mirror finish. Not recommending shine on Gottlieb doors. Sacrilege

Nice, but it won’t take pits out of metal. Legs that are beat up will never look great.

If you want shiny, you need high rpm’s. Buffing with the machine you bought will do the job.

Try using the spiral pad with the white compound.
I've got a box of Eastwood pads and compounds and it's the only ones I use now.
That combination seems to work great for me without using anything else.

The kind of scratches you are working with you need more aggressive stuff.

400 isn't rough enough for the scratches you were facing. You probably need 220. Obviously the lower grit you are using, the more careful you need to be in terms of using anything powered. You also didn't need to go above 2000 grit if you weren't already working towards a mirror finish.

White compound on loose wheels is like plastic polishing. You need a tighter wheel if you are working to remove grain, etc. Green compound on a spiral wheel is my 'goto' for normal pinball polishing. It should take off ball trails and leave a high gloss finish without any additional rework.

600 grit is my goto graining grit if I recall. 800-1200->++ if you are going for that polished finish. But unless I'm trying to go mirror, I generally just stop with the green compound. Now a days I just use red and green scotchbrite pads if working heavy issues, then follow-up on the buffing wheel for polish. Using sandpaper only if I know I have to remove material to hide the defect.

For most parts, I start with green, and if there is still grooves or scratches, start using sandpaper moving your way down until you can see the defect being evened out. Then work your way back up in grit. Then use either the buffer wheel, or you can get green compound dremel-like accessories that you can use directly on the piece.

On that door, I would be moving down in grit until I see the lines break up with my sanding.. then start working up towards 600. The green compound on a mold can REALLY do some heavy cutting on its own saving lots of sanding.

Your photos are a little blurry but it doesn’t look like you are working with bare metal… but something plated with pitting.

That chrome is very dull making me doubt what is going on. I would be approaching plated materials differently then the steps outlined previously. Played materiel you must be less aggressive with as you can burn through it. I don’t really do any chrome myself… but i’d probably would be starting at1000 if metal polish wasn’t getting the results I wanted