Scuffing paint on pre-primed door

I just installed a steel pre-hung exterior door. The finishing instructions suggest "scuffing" the factory primer before apply a finish coat. I understand the purpose of this, and this will probably sound like a pretty dumb question, but what is best for this? Sand paper (what grit/type?) or steel wool (what coarseness?).
TIA
Dan
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Dan wrote:

steel, for fear of rusting. Fine steel wool would be my choice...a primer should have no gloss, so that is an odd instruction. If a flat finish primer, then all that would seem necessary would be to remove any chalking and/or dirt. If using steel wool, vacuum off when finished so you don't leave any tiny particles of steel wool, as they will rust.
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Thanks for the reply. All the things you mention are what I was concerned about. I think I may just wipe it down with mineral spirits & paint it w/o the abrasion. The primer is flat, and no doubt quite thin. I'm imagining that sanding through to the bare metal and subsequent rust is more of a threat to the final finish than the need for adding more "tooth" to the already flat primer.
Dan
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Use a 3M fiber pad....NOT steel wool or sandpaper

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