Ken in Sault St.Marie wants to paint his bedroom but has mildew on the outside corners.
Anywhere where there is wood and no insulation in the walls, like where studs and other framing go right through the wall, we have a thermal bridge that cools the drywall more than where there is insulation. Corners can get colder than any other areas. The top of the walls on the north side have problems too because there are two pieces of wood and no insulation, but there is cold air movement inside the soffits on the other side.
You may have to reduce the humidity in the house, but you could try to warm up the problem area. One way is to insulate on the outside and that can often be done by adding one inch of Styrofoam over the double header inside the sofits, insulating the three inch problem area on the top of the wall.
The removal of a bulkhead often exposes this cold area to moisture and your new renovation has this old problem. Chick on this link to see other ways to insulate the top of the wall.