Bybs,
I don't know how to prevent the nails, or drywall from discoloring, but if you paint, you may find that a coat of "stain blocker" before the top coat will help prevent "bleeding" through.
Terry Love

: Bought house in June of 2001. Installed the Aprilaire Humidifier #568 in our 9 year old house in July along with a new Aprilaire High efficiency air cleaner model 2200. Didn't notice until early 2002 spring but I now have some black marks, like wear the nails are in the ceilings. Looks like maybe a moldish type thing. Had roofers come out and check my attic for proper insulation and venting (whole house roof vent along top)...both of the roofers and one heating plumbing firm said all is well...there are plenty of those styrofoam vent things along the edges and the roof vent is fine. No one could think what might be causing the nails to suddenly discolor...after nine years...then he asked if we had done anything to the house...I told him about humidifier....they suggested that I had it up too high and that what cause the nail markings.......COULD THIS BE? .......BUT
: How do I get rid the marks....Short of repainting? is there a specific solution to use that won't damage the paint? Haven't done anything yet or tried anything cause wasn't sure of what to use.
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: Thank you for your help.