Prep for Cement Board Floors
Solid cement boards. These are often used in wet rooms on floors and walls. Brands include: KNAUF AQUA-PANELS® / NO MORE PLY® / HARDIE® BACKER BOARDS
Make sure the boards are well glued and screwed in place to eliminate any chance of movement. If you are fitting the boards yourself, use a suitable adhesive between the joins, leave to dry overnight and razor scrape/sand the excess off once set. Make sure silicone is not left on the surface to be microcemented. Refer to cement board manufacturer for best adhesive products to use. Counter-sink screw holes.
Boards must be flush with one another, ridges will highlight in the microcement. There must be no voids, even around the bottom edge. Microcement can’t be used to level surfaces/fill voids – it must only be applied as thinly as the grain. The surface must be completely flat. As with all surfaces, microcement will follow the contours of the substrate it is going onto.
You have 2 options for achieving a flat surface:
Option 1: Fill the Joins and Screw holes with MicroDur Base or Suitable Repair Mortar
This is the more basic process. Choose this option for vertical surfaces.
Fill the joins and screw holes so that they are flush with the boards.
IMPORTANT: Leave until dry, minimum 24 hours. If the MicroDur Base/Mortar isn’t completely dry, the lines & holes may ghost through the microcement and be visible in the finish. Once the MicroDur Base/Mortar is dry, use wide, self adhesive skrim tape to bridge the joins in the corners. Apply diluted Primer 100 (with 50% water) with a medium pile roller. Then cut fibreglass mesh and lay it over the whole area, not overlapped, flip the mesh so it curls towards the floor and apply neat Primer 100 over it with a medium pile roller. The Primer 100 will hold the mesh in place.
Option 2: Fill the Joins & Screw holes with Epoxy, Mesh & Sand Blind
This is the more advanced process, which we highly recommend for boarded floors where there is more chance of movement when compared to solid surfaces. These products are included in high performance kits & floor strengthening kit.
Reduce the chance of movement transferring through to the surface by applying Epoxy, Mesh & Sand Blind over the entire floor. The epoxy will also fill the joins and screw holes to level off the floor.
If both walls and floor are to microcemented, use wide adhesive skrim to bridge the internal corner before applying the Epoxy. Use a trowel to apply an even coat of Epoxy over the sub-floor and mesh. You’ll need to ensure you have completely filled the joins and screw holes in the sub-floor and also completely covered the mesh, so that it has good adhesion with the floor.
Whilst the Epoxy is still wet, spread the quartz sand across the surface. You can scatter it by hand, starting at the entrance to the room and walking on the sand into the room. Or by walking on the Epoxy with spiked shoes. The following day, sand any high spots and hoover up all the loose sand.
You will need to prime the surface with Primer 100, you can apply a neat coat directly onto the sand. The Primer is applied with a medium pile roller.