Help Please! Outside corner bead drywall problem

[Loboy48]

I have a 45 deg wall in my master bedroom. It is the only open wall in the room. For about 1 year now I have had a problem with a black streak that starts developing at the top of the base board and gradually working up the corner bead for about 18 inches and then stops. The black streak is the width of a pencil lead, and gradually turns very black, and is only on the sharp corner, It is almost a perfect line about 1/8" wide. 409 spray cleaner or a bleach water mix will remove the mark. It comes back in about 4 weeks. The fix I tried was cleaning the black off, sanding the corner 1/2" to either side of the sharp corners, treating with bleach, appling 3 coats of kilz allowing 24hrs between coats, and then painting to match the room with new porter paint. 4 weeks later it was back. I cleaned it and sanded the paint off down to the kilz. There was no sign off the black on top of the kilz. It is standard drywall construction. I had it tested with a moisture probe and there is no sign of moisture in the wall or on the other side which is a walk in closet. I did the bleach, kilz, and top coat fix twice. Did not work the second time either. Any ideas what this black mark is? Also, other than taking the base boards and corner bead out and redoing the whole joint any ideas on a fix or what may be the cause of this? Thanks in advance for any suggestions.

[Paul Franklin]

Do you perchance have an 18" high dog that rubs against the corner? How about a central vac hose that gets dragged around it?

If it's not coming from inside, it has to be a deposit of some type.

Is there an air register nearby that blows on it? What's below the floor? Any chance of an air leak behind that corner that causes the corner bead to be cooler than the rest of the wall? This can cause deposits to build.

HTH,

Paul

[Loboy48]

No animals, no vac hoses or cords. Have been meticulous in making sure nothing from a outside source is causing this. It is masonary block construction with a 4" concrete floor. AC vent is 7ft away on an opposite wall. I think it is the corner bead causing a bleed thru or something. It does not appear to be mold or mildew, however, I am not certain of that. Also, it is a perfect straight line on the leading edge of the corner bead.

[buffalobill]

cover it with a piece of clear vinyl which extends around it by at least 12 inches. use removable blue 14 day masking tape to secure it. if the problem appears inside the vinyl it may reveal a hidden moisture or mildew or mold source. if the problem appears outside the vinyl install a recording security camera and monitor it until you catch the cause.

Loboy48 wrote:

[Norminn]

I would not use bleach on bare metal prior to painting. Is the bead aluminum or ferrous? Sand down to bare metal, wipe with denatured alcohol, dry, prime with rusty metal primer (Rustoleum) right away.

There is a lot of water on bathroom surfaces, and some will run down walls and doors - seems every bath has warped veneer at bottom of door and, if papered, loose corners at bottom of strips. I may write a thesis on the topic :o) It was surprising, to me, when I last painted my bathroom, finding the ceiling too wet for oil paint to stick, couple of hours after last shower and using exhaust fan. It wasn't apparent until the first brush full of paint just slid across the ceiling.

If AC vent is directed toward that wall, it may create a cool spot that adds to condensation. Interesting little mystery :o)

[buffalobill]

[Goedjn]

If it's a perfect line and never gets more than 1/8" wide, it's almost got to be something behind the drywall telegraphing through. (A metal corner bit thats one or all of cold, magnetic, or got an electric charge on it, attracting dirt.) Put some tape there and see if the black stuff is on the room side or the wall side.

Sounds like a good place for some 1/4 round molding, if you ask me, although I might be tempted to drill into the corner to see what's there, first.

--Goedjn