What's best to remove....
 

What appears to be oxidation or some kind of build up after heavy snow season. I tried clay bar and a mild swirl cutter/polish, but had very little luck. Any suggestions?

Did you try the swirl cutter by hand? You might need to use a buffer like the PC to remove that. If the PC/swirl cutter doesn't work you will need to move up to something with a little more "bite" like Menzerna IP or SIP.

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Yeah i put it on by hand and it was a respectable brand, one of our sponsors. Worked fine everywhere else except in one spot. Can Mezerna be applied by hand, i don't have a pc?

It can be done by hand but it will work you to death. You will not get the results applying by hand that you will with the buffer. Where are you located? Maybe there is someone close to you that has one? If not I would just keep working the area that is giving you problems. Once you are satified with the paint, put on a good coat of polish/wax. That will help protect the paint.