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Selecting the appropriate primer

Selecting the appropriate primer
Adam Jones

Published by Adam Jones - Technical Consultant

Adam has been part of the Bostik team for more than a decade. Starting as an apprentice laboratory technician, he spent a number of years supporting performance testing and new product developments within the flooring range.

Priming is, at least in my experience, an often misunderstood and under-appreciated aspect of subfloor preparation. What some installers see as an extra, unnecessary step can be critical to the long-term performance of the installation.

“A floor primer enhances the flow characteristics of a smoothing compound by preventing rapid drying. Primers should be used on absorbent subfloors to seal the surface, helping to prevent pin holing, which can occur as air is released from the subfloor.

On non-absorbent surfaces, a primer will promote adhesion and aid workability of the smoothing compound.”

However, with so many different solutions on the market, it can be confusing selecting the correct solution for the installation. In this article, I will discuss the common primer options available in the market and how and why they should be used.

Acrylic primers, often referred to as universal or multipurpose primers, are the standard go-to product for most common situations. These products can be used on many different substrates, and also at different dilution rates depending on the required effect. A diluted acrylic primer (typically 1:2 with water) will readily soak into, and act as a sealer on highly absorbent substrates such as screed and concrete, preventing the subsequent application of smoothing compound from losing its water into the screed too quickly, thus improving handling during application and enabling the retention of a suitable wet-edge to allow mixes to blend seamlessly. They can also be used neat (undiluted) to act as a bond enhancer on dense substrates, such as an epoxy DPM. In these situations, it is not necessary for the primer to act as a sealer, however these surfaces are difficult for smoothing compounds to bond to and so the primer will assist with this. One common misconception when it comes to the use of acrylic primers is to use them neat on absorbent substrates, expecting that not diluting the primer is giving an even more robust coating. This is not the case, and can often cause the opposite effect, where the primer is too thick to effectively soak into the substrate, resulting in primer just sitting on the top layer of aggregate. In situations where a porous substrate requires a neat acrylic primer application, it should always receive a diluted application prior. A limitation to standard acrylic primers is that they are typically only suitable for applications of smoothing compound below 10mm. Once this thickness is exceeded, you will have to consider a higher specification primer, such as the types I will discuss below, which can cope with the added strength/strain.

Gritted primers are generally acrylic-based products, but are of a higher specification and are designed to cope with demanding installations. They are a good solution when a smoothing compound is required over a substrate that is dense and smooth, for example existing epoxy/Polyurethane (PU) coatings, terrazzo, metal, and some types of tiles, including ceramics or porcelain. The added aggregate effectively increases the surface area and vastly improves the mechanical key to the substrate you are bonding to.

Epoxy/PU primers are high-spec and extremely robust products. These types of primers are ideal for the preparation of calcium sulphate screeds, as they prevent any chance of a reaction taking place with subsequent applications of cement-based smoothing compounds. Without the use of an epoxy/PU primer on a calcium sulphate screed, you may be faced with the production of ettringite, a crystalline layer between the smoothing compound and screed, which inevitably leads to bond failure.

These types of primers also do a great job of eliminating absorbency, meaning if sensitive coatings are required (floor paints, heavy duty smoothing compounds that will remain uncovered and used as a final wear layer etc.) they will not be affected by the background. This also means they are great for timber substates, as no water is being added into the subfloor from either the primer or the compound application, which can sometimes influence the timber to swell or move. Certain epoxy/PU primer products also carry extra benefits, including being used as a moisture control system when applied in multiple coats. Furthermore, these types of primer sometimes have the ability to consolidate weak/friable surfaces, allowing for some weak upper layers to be strengthened during the priming process.

So, in summary, there are very few situations where selecting and using a suitable primer would not benefit the fitting, finish, and longevity of your flooring installation.

This article should be considered as general guidance as, naturally, there will always be variations between different manufacturer’s products and no two should ever be considered truly like-for-like.

Should you have any uncertainties on best practice for your installation’s needs, it would always be beneficial to consult with a Technical Representative from your chosen product manufacturer.