Theory Behind Cement Based Patching Compounds
Why would you use a cement based product to patch the joints of a cementitious substrate like blue board? The impact of typical settlement, subsidence and expansion and contraction, combined with the rigid nature of this type of wall panel can be most evident at the joints.
Using a cement-based patching compound such as RSA’s Set & Prep or Rockcote’s Patch & Prep to set the joints with a quality non-adhesive fibre reinforced mesh can reduce these effects on aesthetics. This is because, once the normal, initial movement occurs on a new building or extension the stresses are restricted within the grid of the mesh. Over-coating with a flexible acrylic based product such as RSA’s Flexirender or Rockcote’s Polymer render will then bridge any small hairline cracks and exhibit the malleability function required to cope with any further ordinary stress factors.
In contrast, whilst a wet patching compound (that comes in a bucket) has the flexibility to go with the building movement it can also be pinched between the two panels compressing on either side of the joint resulting in ‘peaking’ of the joint. This can appear as a bump running out of and along the joint and can be difficult to disguise and even exacerbate the normal effect of side light / glancing light.
The cementitious, polymer modified bagged, adhesive products are specifically designed & proven products for jointing systems.