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Don’t Let The Subfloor Be Spoiled For Choice

Martin Cummins on choosing the right smoothing compound

THE enormous choice of smoothing compounds available to the modern flooring contractor can be quite daunting. The technical needs of the substrates along with the contractor requirements and the needs of the floor covering can often have significant conflict.
When the opportunity to spend time with contractors comes along I always try and break down the conflict and have a few key pointers to help you find your way through the minefield. Some are very obvious, some perhaps not.
Try and avoid ‘hard over soft’: If the existing subfloor, adhesives residue, previous smoothing compound are of low inherent strength then the application of high strength smoothing compounds can be problematic.
As a higher strength product cures and ‘tightens up’ it can exert strain and stress on weaker underlying materials. This can result in cracking, shrinkage and can even cause debonding.
If in doubt as to the strength of the existing floor then move towards bag and bottle products of lower strength…coincidentally they often are high polymer systems giving excellent adhesion also.
Consider pumping larger jobs: If there is a new floor, or even a well prepared existing floor of suitable strength that is open plan and requires several tonnes of smoothing compounds consider using a water mix and having it pumped. The correct use of pumping equipment offers a superior floor finish and reduces site time and downtimes for areas. There are many specialist pumping contractors and also the purchasing of a pump system can be cost effective if this is a typical type of work for your business.
Still look to prime: There are compounds available with excellent adhesion that will bond to a variety of ‘difficult’ substrates such as old adhesive residues, asphalt, terrazzo and surface applied DPMs.
However, the claim of ‘no need to prime’ does not take into account the fact that you will still get pinholes on absorbent floors and that workability of the product will be greatly affected if you don’t prime.
Also, some incompatible substrate REQUIRE priming. The biggest concern is when going over epoxy DPMs where manufacturers talk about not needing to prime provided…condition, condition, and more condition. Just use a bonding primer to take out all the concerns; it really doesn’t add loads to the job price but can save headaches and later uplifts!!
Assess all the likely site problems beforehand: If you know you are going to be on a building site in winter then you need to appreciate all curing and drying will slow down. Indeed, allow temperatures to drop too low and you can permanently affect the curing.
Using faster cure and set products can help reduce the delay times but obviously will cost more than standard compounds so let the main contractor know the limitations…a hard task but I am sure you will always get support from the manufacturer.
Also, sometimes there is a working programme that needs the areas to receive foot traffic soon after application, whether it be to let trades continue working (so very annoying) or to allow the clients back into their properties. Again rapid setting and curing products are favoured for this.
Understand what moisture tolerance is: Many contractors want moisture tolerant products.

Manufacturers formulate products that will continue performing even under damp conditions making them ideal for pre smoothing rough floors prior to DPM applications.
Also, they can be used where subfloor moisture is not being checked by a structural DPM but there are limitations. The creation of hydrostatic pressure can cause floor covering to ‘blow’ and pull up smoothing compounds even if they are moisture tolerant.
I would always advise that large spans of floors or floors where the lying land has a potential high water table (i.e. near rivers, low lying land, flood plains etc) should have the application of DPMs direct to the substrate rather than smoothed first.
The minimising of the number of layers is again an adage I would always try to follow. I hope these pointers are of help. Happy smoothing!!

Martin Cummins is Ultra Floor technical sales manager

T: 01827 871871

This article has been reproduced from the Contract Flooring Journal website. You can find them at