Patching compounds are designed to fill holes, cracks, grooves and damaged areas that are too large for a self-levelling smoothing compound to fill efficiently.

Hairline cracks are generally not a problem, but anything larger than about 5 mm should be filled with a patching compound.

Some patching compounds are cement-based mortars and are applied with a putty knife or trowel. Others are made from epoxy or polyurethane, and can either be applied like a mortar or injected into the crack.

You should never use gypsum-based products, like water putty or cornice cement. These products are high in alkali and can destroy adhesives, they swell when they get wet, and they don't have sufficient long-term adhesion to last the lifetime of the floor.

Set out below is the general installation procedure for a typical cement-based patching compound.

Preparation and installation procedure

1. Make sure that the substrate is dry, firm and free from dust and any contaminants. Depending on the product and the type of substrate, you may need to prime the surface before applying the patching compound.
   
   
2. Mix the powder with clean water in a mixing container to produce a mortar.

3. Apply the mortar with a trowel to the holes or damaged areas. Push the mortar into the void and leave it 'proud' (sitting slightly high) of the surface.
   
   
4. After about 15 minutes, trim off the excess mortar and finish the surface with a wet trowel or sponge.

5. Leave the mortar to harden. Under normal conditions, this may take about one hour.



6. Apply the smoothing and levelling compound before the mortar dries. Note that if the area is allowed to completely dry before the levelling compound is installed, you may need to re-prime the surface.