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Smoothing compounds as wearing surfaces

Smoothing compounds as wearing surfaces
Martin Cummins

Published by Martin Cummins - UK Technical Support Manager, Bostik

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential.

In the contract flooring industry, most installations are for a new or replacement floor covering. The smoothing compounds required below them won’t be subject to direct wear-and-tear, site abuse etc., and that’s why we refer to them as underlayments – they lie underneath the floor covering.

The benefits of our range and other manufacturers’ are such that they can be developed to go over different subfloors, can have moisture tolerance for use under DPMs, lots of polymer for use over old adhesives and even fibres to give additional crack minimisation characteristics on heated floors or floors with vibration. The one key thing is they’re preparation materials for use before floor coverings.

I’m well aware that sometimes on sites the smoothing compound won’t have the floor coverings laid for extensive periods after application. They may therefore be subject to site traffic, damages, contamination etc., and often visually appear to hold up very well in these instances, which is testament to their qualities.

However, cracks may occur owing to further dehydration, post expansion may occur if they’re rewetted, or dusting up can occur if they’re treated harshly. Therefore, offering any sort of guarantee for the product when it’s used outside its design parameters isn’t easy, so there’s always an element of risk to longevity.

“Offering any sort of guarantee for the product when it’s used outside its design parameters isn’t easy.”

I have had many a conversation with enthusiastic sales reps, flooring contractors and also architects, clients and through our Bostik helpline for general builders and members of the public as to why we can’t recommend using underlayments as the final floor surface.

There’s always the anecdotal story of someone who has done it and it’s fine. Yet they still ask the question, so I suspect maybe it hasn’t actually been fine? I’d like to explore with you the differences between the requirements of a smoothing underlayment and those of a finished wear-surface product.

As a finished wearing surface, there’s the potential for a substrate to suffer from significant ‘abrasion’ from turning circles of anything from heavy pallet trucks in warehouses to the twisting and turning of a tool box in a garage area.

Wearing surface products have vast amounts of polymer in them, along with high surface compressive strength in comparison to underlayments to enable them to achieve an abrasion rating. The abrasion resistance is more important than the compressive or flexural strength for this type of product. This is effectively a classification for suitability to not have anything further on top of them and for them to still perform, not dust up and to last a long time. Your average bag ‘n’ bottle won’t do this.

The next thing they need to have is extremely accurate shrinkage control mechanisms. The balance of small additive materials to enable the products to cure and tension up, but at the same time not shrink back, isn’t easy to achieve. Its criticality when underneath a floorcovering is nowhere near as important as when it’s the visual aspect you see from above. Hairline crazing under a carpet tile – so what?

Hairline crazing on a finished surface can be an eyesore or a potential problem for dirt gathering and long-term cracks developing.

“Hairline crazing on a finished surface can be an eyesore or a potential problem for dirt gathering and long-term cracks developing.”

Third, the cement, sand and fine additive combinations also have to be perfect and, when mixed up with the correct amount of water, the system needs to dry to a consistent colour. Over-watering these products to achieve easier handling will undoubtedly cause some surface separation and leave a non-uniform, discoloured finished surface. The separation will also result in a difference in abrasion performance if fine particles are ‘floated’ to the surface.

The last important thing to consider is the function of the wearing surface. If it’s in a dry environment or if aesthetics aren’t as important as performance, then they’ll often be left simply as they are after curing. However, it’s important to understand that when cured they’ll still act as an absorbent surface.

Water spillages or indeed other liquids will soak in and stain the floor.

Oils will also soak in and be difficult to clean off making them a potential hazard. In any of these situations the use of a protective coating is recommended. This is another key reason you need a consistent surface finish for coatings to perform correctly.

The protective coating can range from a simple, clear water-based coating to prevent liquids soaking in and maintaining the visual aspect. They may also be a thin-coloured stain or coating to give a different appearance.

In fact, there are some wearing surface compounds which have slightly different variations in finished colour to give an element of design which can just have a clear coating. In more commercial applications this will extend to suitable protective paints or thicker coatings such as low- and high- build epoxy coatings.

This can offer seamless hygienic floors with very good, long-wearing benefits that will last as long as the paint does and can be re-coated (with preparation) at later dates. Always check with FeRFA if you’re required to put a resin floor finish as this is a skilled job and you’ll need to know if the smoothing compound specification chosen is suitable for the resin system being used.

There will often be a minimum compressive strength for the substrate at the time of application of the resin, so you may need to leave the wearing surface some time before recoating.

Now, if we go back to a standard underlayment and ask does it have all the requirements of a wearing surface, what are the answers?

  • Abrasion resistance: No, not required
  • Precise shrinkage control: To a much lesser degree than wear surface products
  • Consistent surface drying colours: Some better than others, but not a requirement
  • Suitable to receive coating: Not normally recommended

So, if you need an underlayment before a floor covering, stick with what you presently do. If you’re leaving an underlayment exposed for weeks/months before laying flooring, don’t expect a perfect surface by the time you lay flooring.

“If you need an underlayment before a floor covering, stick with what you presently do. If you’re leaving an underlayment exposed for weeks/months before laying flooring, don’t expect a perfect surface by the time you lay flooring.”

Finally, if asked for or if offering a wearing surface floor using a smoothing/levelling compound, choose one that’s designed to do exactly that! It’ll be worth it in the long run.