Plaster Preparation for Compressed Earth Block Walls

Plastering provides both protection and beauty to Compressed Earth Block (CEB) walls. Regardless of how good, or perhaps bad, your wall is built and performs; it is the plaster that you and your visitors will see. Correctly preparing CEB walls for plastering is critical to the longevity and quality of the plastered wall. The instructions below provide steps for the successful preparation of CEB walls for plaster. It cannot be over emphasized that failure to follow these steps will result in plaster failure. Furthermore, these instructions do not cover plaster recipes or application techniques that are equally important to successful plastering.  

“Keying” refers to uneven or rough surfaces that allow the plaster to mechanically bond (i.e. grab) the wall. This is provided by grooves, scratches and indents created in the CEB surface or through the application of lathe, stucco wire or burlap. Keying can be created when the CEB is manufactured by manually creating grooves or scratches with sharp hand tools, or created later with a grinder, saw, hatchet or the claw end of a hammer. We recommend providing a minimum of two scoring marks 1/8” to 1/14” in depth running the length of the block face.  A diamond tipped masonry blade attached to a grinder is best for accomplishing this depth.

Remove all broken edges, loose mortar and dust from the block surface. This can be accomplished with a wire brush and 4” dry wall blade. The wall can also be sprayed down with pressurized water to clean the dust off.

The wall must be wet before plastering—not soaking wet but visibly damp. We strongly recommend that the wall be sprayed down with pressurized water several hours or up to a day in advance of plastering. The wall will need to be sprayed periodically in order to maintain the desired dampness both before and after plastering. Stabilized CEBs will require more water then unstabilized CEBs.  If you notice the blocks beginning to erode or even expand, you are adding too much water.  This is especially the case with unstabilized blocks.

One of the top reasons that plaster fails is because the wall was not protected before, during and after plastering from direct sunlight and hot dry conditions. These conditions dry the plaster out before it has time to set and bond to the wall. Provide tarps or burlap to reduce direct sunlight and help keep the wall damp. For walls that are in the shade, plan to plaster on a cloudy day. For walls on the south and west facing sides, plan to plaster once the sun has gone down.         

Earth Blocks showing keying on surface during wall construction.  Note the cool radiant tubes also.