Ok, heres one. We bought our house about 13 yrs ago, and while installing a range hood and using a stud finder - it didnt work. Curious, I decided where I wanted the hood and cut a hole through the drywall. SURPRISE - our house has no wood studs, but what looks like a "U" channel steel stud in all exterior walls. Ok, so it holds up well in a bad wind storm, BUT how do I stop the mold or blackened look from bleeding through the paint? We have tried numerous types and brands of paint and even used Kilz and varnished the stud area in one room before painting. But low and behold, within 2 yrs you can see where the studs are, as a black mark appears every 24" running from ceiling to floor. Any thoughts?