Using Grout to Fill Bugholes

Question:
When I fill bugholes in my countertops with a slurry, it tends to shrink and stays dished after I hone it off. How do I prevent this?

Answer:
When slurry, or cement grout, is used to fill in pinholes and larger voids in a concrete countertop, it’s often applied with a scrubbing action to force it down into the small pinholes. Once hardened, the excess is honed off to leave a flush, smooth, void-free surface. However, if the grout is too soft when honed off or the grout is not applied correctly, a dished effect and other appearance problems can result. Here are some dos and don’t when mixing, applying and honing grout:

• – Keep the water-cement ratio low. Grout is a form of concrete, so a low water-cement ratio will help make the grout harder and stronger.
  
  
• – Add superplasticizers, accelerators or pozzolans to the grout. These additives will also make the grout harder and stronger and shorten the waiting time between application and honing.
  
  
• – Add a fine-grained filler, such as cenospheres (tiny hollow ceramic spheres), to help reduce grout shrinkage and give the grout some “tooth” when filling small pinholes.
  
  
• – Don’t make the grout too fluid. The smearing action used when the grout is scrubbed into the surface is less effective, so the pinholes don’t get completely filled. What’s more, the thin cap of grout that does cover the pinhole often gets removed when the excess grout is honed off. If the pinholes are filled with water, the same thing happens.
  
  
• – Don’t apply grout with a trowel or a rubber float. These tools are good for filling big holes and voids, but they won’t adequately force the grout into the smallest pinholes. Vigorous scrubbing with gloved hands seems to be the best method for that.Don’t hone off the grout when it’s too soft. If you can scratch or gouge the grout with a fingernail, then it’s far too soft for honing. Honing with diamond pads (around 200 to 400 grit) can cause softer grout to erode, leaving a dished surface. For the best results, wait and hone off the grout after it becomes rock hard. It will then mill off smoothly, leaving a flush surface.
  

Applying grout to fill pinholes and voids in countertops is a skill
that requires practice, patience and proper mixing.