Had a couple questions on what the heck I'm showing in the video.. Haha... so let's clarify..
We were blocking out the primer on the roof and came across a few low spots. These were low enough to be seen in the paint if left alone, and yet mixing and sanding any filler "repair" would have taken longer than some simple bumping of the low area.
Since I don't have a Bullseye Pick, we thought to use some gentle hammering with a crowned body hammer beneath into a small shot bag on the top (outside) of the roof. We have some 65 lb pull rare earth magnets that we typically use for our paper patterns, and this showed to have enough strength to pull through the roof skin. So we placed the magnet in the center of the low area, and in order to locate on the underside, would skim across the roof from beneath with the hammer face to see where the pull was strongest. The location was then marked underneath with a sharpie, magnet removed and replaced with the shot bag, and the area was tapped into the shot bag until blocking showed the area where it should be.
On one of these "marking with a sharpie" efforts, there was a slight bit of dust on the bottom side of the roof, so I gave it a wipe with my hand to displace the dust and instantly saw the outer circle of the magnet. This was much more accurate than using the magnetic pull test, so we changed to this, and the metal bumping effort worked well. We surmise that this "dust" that clings to the magnetic field is likely sanding residue from the primer, or sanding residue from dressing welds inside the car, or both. In any case, it worked well to help transpose the low spot locations to the bottom side for more accurate hammering.