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Lime plaster over existing plaster and block

Hi, I am working on a fire restoration. The home is concrete block on the exterior walls that were previously finished with plaster. It looks like the block had been coated with an adhesive, and I have had had to grind down to the scratch coat to remove the heat and soot damage. The "key" has been broken in many spots-which are now down to the block. There is some remaining wire, but only around window and door frames. I have ordered NHL and want to mix it with different grades on sand to produce two or three coats. My biggest question is can I get this new lime plaster to adhere to the previous plaster of unknown origin and block. I have heard of "knocking up" or 'throwing'" a first coat- is this necessary? I have never plastered before. Any thoughts?

Asked by Lee Favro
Posted Feb 13, 2013 7:46 PM ET

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Lee,
I'm not a plaster expert. Perhaps a GBA reader will chime in to answer your question.

In the meantime, here is a link to a web page with information that might help you: Cement Plaster Over Concrete Block Walls.

As you might imagine, Portland-cement-based plaster (stucco) is more common than lime-based plaster. Here is another article that might help you: Make it Stick: A Common-Sense Way to Apply Stucco to Concrete Block.

Answered by Martin Holladay, GBA Advisor
Posted Feb 14, 2013 5:19 AM ET

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