Since we have finished our home about three years ago, we have noticed a black discoloring forming on the walls on the interior. The black discoloring is almost soot-like and is prominant where the trusses and wall studs meet. I do not notice it except at that juncture or following the wall studs. There are places in my garage where I can see where every wall stud is. These walls also show nail heads as dark blackish circles. Any ideas what this is?
I ran up on something like this a few years ago and it was mildew on all the areas of joint compound. To test if it is mildew, just apply some bleach to one of the dark spots and if goes away it is mildew. I fit is mildew you will need to wash all walls with a 25 percent bleach and 75 percent water solution to kill the mildew . Once this is done you will need to prime the walls with Zinsser BIN primer and finish with the topcoat of your choice. This primer does not get rid of the mildew but is good insurance in helping to keepit from coming back.