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.
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.
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.
is there a humidifier or ionic machine in the room?
install some decorative trim or a shelf?
if the kilz is the original formula it sounds like a good job. if the
kilz 2 latex base is used could it be rusting an iron substitute corner
radio shack has an electronic temp/humidity indicator to see what the
room humidity is.
in the absence of a perfect vapor barrier and insulation, moisture can
travel thru your described building materials.
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.
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.
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)
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