Yeah, you are going to have to sand the entire thing in order to get a consistent finish. You bought that from a vendor? Is that normal or did you get a b stock item? If that is not normal I'd ask for a replacement.
Instead of polishing, can it be cerakote finished? I have no experience with cerakote. Some vendors listed below
https://highcountryarmory.net/
https://www.actioncerakoteandlaser.com/index.html
Per Ardua ad Astra
The effectiveness of Dremeling 3D Printed parts depends on the material used to print it and if "Hollowing" patterns under the surface were used to make the part lighter or not consume so much material. Even if its a dremelable material, you will need to use a low speed and a light touch to keep the plastic from overheating. The few times I have dremeled on 3D Printed parts I had to use a fairly coarse sanding drum bit to take off the bulk of the material in a manner that would easily "Dig" the surface later off without getting the plastic too hot. Then I would go back over it with a fine grit flapper wheel to smooth it out.
Wet sanding it with graduating finer grit paper would be the least risky way to smooth out 3D printed parts. But that can take a long while.