A parge coat is a thin masonry coating that is applied over interior and exterior masonry block and stone walls. Kind of like frosting on a cake. It is most often seen on above grade foundation walls and the interior of basement walls. I’ve inspected hundreds of homes where the parge coating was not properly applied and loose and cracking and falling off in chunks, sometimes as soon as the same year it was applied.
The purpose of a masonry parge coat is to protect the materials making up the wall, the stone, brick or masonry block and the mortar in the joints that hold the wall together. The parge coat forms a water resistant, protective barrier that helps prevent water penetration into the building materials that can cause moisture damage or spalling deterioration.
As in many things good preparation, using the proper materials and the correct technique make a world of difference. A good parge coat job will be practically maintenance free and last for many decades. A poorly applied parge coat could last less than a year, looking worse than before you started and need almost constant maintenance and repair some cases.
For a quality parging mix its best to use a mix of about about 3 parts sand to at least 1 part portland cement. You can add 1/4 to 1/2 part lime powder to make it a little easier to spread or apply. This is mixed with enough water to get a nice consistency, not too runny. If your not experience be careful to not add too much water, you can add a little at a time. I like to use a little bit of liquid adhesive or acrylic admix, along with the water, using about 10% – 15% as much as water.
Adding the adhesive or acrylic admix will help it to be a longer lasting repair. The admix looks like milk or thinned out Elmer’s glue, and it is sticky. It’s also a little expensive, but worth the extra investment. It helps the parge mixture to get a better bond or stick better to the wall and makes the finished parge coat a lot more water resistant almost water proof.
Before you mix your parge mix you want to clean up the wall, power washing or a hose and brush is good to clean off dirt and loose particles and chunks. After your parge mix is ready to go, mist a section of wall with water to get it damp – usually changes the color. As soon as the wall was surface soaks up the water, no drips apply your parge coating. Usually wetting a section of wall ahead of where you are working has it just about ready when you get there.
For some more information, found a pretty good webpage on this subject with pictures and detailed info at the: New York Renovator