A bare metal area surrounded by body filler after sanding indicates:
B) a low spot in the area being sanded.
When sanding body filler on a panel, a bare metal area appearing means that the filler has been sanded down to expose the metal beneath it. This typically occurs in areas where there is a low spot or depression in the panel that required more filler to level out the surface. As the sanding process continues, the high points get sanded down first, leaving the low areas with less filler and ultimately revealing the metal beneath the filler. This indicates that more attention is needed to fill and level the low spot properly before painting or finishing the panel.
