On my painted drywall on only the exterior walls of my house I'm having dark spots appear where I securedthe drywall with screws. I've washed & repainted the walls, but eventually they appear again in time. The interior walls don't have this issue, only the walls that have an exterior side to them. Is there something about the cold studs attracting dirt & dust to the screw heads imbedded in the studs? Before the drywall was put up 1" urethane foam boards were used to sheath the studs. This doesn't happen on the ceilings, but these are 2 x 10's rather than the 2 x 4's which are the exterior walls.
Is there something I can do to correct this problem?