How do I sand this down!?

MileHighDart said

Ummmm yeah, right, and while your there pic up some new muffler bearings too. And be sure to ask for the green ones, not the tapered roller style. Better performance and less noise.

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Hate to tell you, but he's right! I used to sell a product called "liquid sandpaper" made by krylon or duplicolor I think. It was meant to be used in hard to reach areas like door hinges by softening and etching the paint so primer will adhere better. But I'm going back almost 20 years ago.
And if you need a muffler bearing, use Nissan part # 20695-8H310 (Look at the package)!!