The purpose of a primer is to improve the adhesion of the levelling compound to the substrate.

Sometimes they contain additives designed to address certain conditions in the subfloor - such as a sealant for very porous concrete to prevent air bubbles from rising up through the underlayment.

However, it's important to note that primers are not designed to be moisture barriers, even if the product is described as a 'sealant'.

Unless the product has been tested in accordance with AS 4858 and has satisfied the required 'moisture vapour transmission rate', you can't depend on it to stop rising damp or other moisture transmission problems.

Nonetheless, some primers are designed to retard - or slow down - the movement of moisture, so they can help to protect adhesives and floor coverings from a small amount of moisture rising up through the slab.

Underlayment manufacturers normally supply their own primers to go with their range of levelling compounds. They may also recommend other products for specific subfloor issues, such as dense or smooth substrates, or very absorbent surfaces.

Most primers are water-based and can be applied with a roller, brush or spray gun. The general installation procedure for a water-based primer is set out below.

Preparation and installation procedure

1. Make sure that the surface to be primed is free from dust, dirt, oil, grease, curing compounds and any other contaminants.
   
   Check that the moisture content is appropriate for the primer being used and the underlayment that will be applied over the top.
   
   
2. Pour the primer into a container, ready for application, if it is designed to be used undiluted.

3. Apply the primer to the floor using a roller, sponge or brush. Normally one coat is sufficient, but you may need two coats on very porous substrates.
   
   
4. Allow the primer to fully dry before applying toppings or other products. This may take from 30 minutes to several hours, depending on the product, substrate surface and the weather conditions.