How do I sand this down!?

Get some napa degreaser in a pint and dillute it based on instructions and soak the area and rinse with lots of water. Then like others said, get some 320 sand paper and some red scotchbrite pads and put the spurs to it till it is all scuffed dull. If you have tar based undercoat or goop, dip the scotchbrite in the degreaser and scrub till its totally gone. Finish up with another shot of degreaser / rinse / dry. Then use ecthing sealant primer followed by paint.

Otherwise you can get small sand blaster and blast it after scrubbing it free of grease and undercoat.

Degreaser and good quality primer - paint

http://www.clean-rite.com/purplepower_industrial_strength_cleaner_degreaser.html

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