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The paint that is on those new parts is basically a primer anyway so you shouldn't need to primer them, I think 1200 is a bit light, you might try 600 or 800 if you think 400 is to harsh. Try to avoid sanding to bare metal if you can avoid it.



And don't forget to wipe down the entire surface after with a cloth dampened in mineral spirit to remove any debris or dust after your sanding. You don't want anything left behind that will interfere with you paint adhering to the subsurface.

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