The old guy who taught me a lot, told me this about 2 days into my mentorship with him when he smacked on the back of the head

as I was sanding with al-ox paper over a leaded seam on a nearly new Fox body Mustang that was in the Shop. I didn't do it again.

Later I found out the explanation for why you don't do it. Back in the day when lead was the predominant body filler, guys never sanded it at all. They filed it. Start with a fairly coarse body file about 8-10 TPI then finish with a fine body file around 20 TPI. Then prime. How it also was done at the factory.