Techniques To Achieved The Desired Finish
Martin Cummins on ‘smoothing compound’ or ‘screed’? – part 2
LAST month I discussed certain flooring terminology and clients’ confusion over screeds, levelling compounds or smoothing compounds. I concluded referring to levelling and smoothing compounds as materials used to give a finished surface ready to receive decorative floorcoverings.
This month I will explain different products and applications on the finished surface. Levelling compounds vary widely in make up. Different properties include rapid drying, flexural and compressive strength, polymer modified, 2 part , latex, SBR etc.
These are all factual information on specific products, but none really a guide on the finished surface. Why so?
The minimum expectation is that a professional flooring grade smoothing /levelling compound is suitable for all installations, assuming that all give the same finished surface.
This is not strictly true. Some higher strength floorcoverings require high strength smoothing compounds, particularly in areas subject to thermal changes or that require stronger or more flexible products.
The finished surface is primarily affected by what contractors do or don’t do. By using the product correctly with due consideration to priming, you can achieve a floor smooth enough to take the thinnest vinyl. However, incorrect use or inadequate preparation can lead to a ridged, rippled and weak surface, potentially unsuitable for bonding too.
What are the critical application techniques to give the desired finish?
First, smoothing compound selection helps considerably. Traditional water mix smoothing compounds are thought to give a far superior flow and with laser levelling, floors of SR1 regularity as smooth as billiard tables (as the saying goes) can be attained. We at Ultra Floor have tried to come as close to this as possible with our bag and bottle products. Technology has moved on so you should expect better flow and smoothing characteristics also from ‘latex’ products.
This leads me to what I suspect is done against manufacturers recommendations.
n Avoid adding extra liquid: This may make the product more fluid, but it is only trying to either overcome a poorly manufactured product or make it easier on application. In reality it disturbs the cement, polymer and filler ratios in the (quite complex) formulation.
The result is often a weak surface (referred to as laitance) which does not readily receive adhesive and also can simply loosen as the compound fully dries. It also compromises the strength build up in the compound.
It can greatly affect the drying rate, so you either have to delay fitting or fit too early, potentially causing a failure down the line. Another way of causing this ‘laitance’ is by over-zealous use of spiked rollers.
n Use spiked rollers sparingly: These rollers were introduced to ‘marry’ adjacent mixes to give a smooth transition. By using them across the compound you can improve the trowelling action to give a smoother surface, but continually doing this in a ‘hovering’ manner can disturb the matrix and give the same result as over watering.
Using spiked rollers as the product is going through its early cure can create a stippled finish. This differs from the pinhole effect often found, leading to the next piece of advice…
n Prime absorbent floors if laying resilient flooring: The movement of air from an absorbent subfloor into a curing smoothing compound will cause pinholes (wormholes) and /or small blisters.
Wormholes are due to air passing through and breaking out, but the smoothing compound is then not fluid enough to flow back into itself. Blisters occur because the levelling compound is building up viscosity on the floor and the air cannot pass through.
Both these cause a weaker surface than desired and while this maybe OK for a carpet or ceramic application, it is not for resilient flooring.
Many flooring contractors still use rubbing down or feathering of the finished surface as a rule of thumb to give the preciseness of finish required. This shows your dedication to provide as good a floor as possible.
Follow the advice above and you’ll likely have fewer problems.
Martin Cummins is Ultra Floor technical sales manager T: 01827 871871 www.ultra-floor.co.uk
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