Cleaning Concrete Block Walls

Eric Sygnatur owns a Hyattsville, MD townhouse located just outside of Washington, DC where it's very humid. He's got a strange stain on the exterior concrete block.

"Tim, my townhouse is five years old. There is a stain on some of the masonry that I thought might "wash away" or fade with time, but it hasn't. Any idea what these drips are - perhaps some kind of sealer?

Here's Eric's wall. You can see a great clue as to the cause of the stain on the block just behind the light fixture. Photo credit: Eric Sygnatur

More importantly, how can I remove it?  Thank you so much for your time!"

Here's my response:

Eric, I'm convinced it's a sealer that was sprayed on the porous masonry. The clue is the splash pattern behind the light fixture.

The sealer picked up / dissolved, I think, pigment from the light-colored split-faced block used in the construction.

I'd try to use some mineral spirits on the stain just behind the light fixture as a test to see if you can reverse it. Some sealers use a hydrocarbon vehicle to deliver the solids into the masonry.

Mineral spirits may put the pigment back into solution. Apply a SMALL amount with a toothbrush and scrub lightly. Dab with a paper towel, a white one, to see if you get a brown stain on the towel.

If so, then you might be removing the offensive pigment. Let me know what happens.

NO SMOKING while you're working with mineral spirits! BURN the paper towels immediately after getting them wet with the flammable liquid.