Dr Schutz - We Care About Floors
09 Nov 2021

Skim-fillers: Filling in the Cracks

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We all know the importance of preparing walls and ceilings before decorating them to achieve that perfect finish. But did you know that filler is increasingly used for more than just patching up holes, gaps and cracks?

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Quality filler brands now offer ‘skim-fillers’ designed for plastering whole walls and ceilings, and some professional decorators even take to skim-filling entire properties. In this article, Toupret’s Technical Manager, Adrian Smith, advises as to how he uses this method, and how you can too.

Fillers are made up of compounds with additives that give them unique properties, depending on the job they are designed to achieve, such as quick drying, high adhesion, flexibility, extra resistance or breathability in damp areas. This makes them a valuable solution for a diverse range of applications.

Fillers are also highly versatile when it comes to where you can use them. There are versions for use on many different surfaces, from plasterboard to cement and concrete, painted surfaces or wood. There are even skim-fillers designed for exterior use, so you can repair or cover masonry and rendered surfaces.

The versatility and ease of use of a filler are also making it increasingly popular as a solution for preparing plasterboard for decorating. Where the mounted boards meet, they should be taped and jointed together using filler to create a smooth surface. The filler can then also be used to skim across the entire surface to create a perfectly finished wall or ceiling. And, thanks to the qualities a filler may possess, which traditional plaster does not, this can often be achieved in one coat, without the need to prime the surface, significantly reducing the drying and waiting time.

With so many fillers to choose from, it’s essential to ensure you pick the right one. Consider the depth you wish to fill to, the type and quality of the surface you’re covering, and whether you want to use a powder or ready mixed formula. A ready-mixed filler might be more convenient, but you can use a powder filler for thicker, deeper filling. It will also set faster, and quick-drying properties, in some versions, means they’re ready to paint in three hours.

Once you’ve decided which filler will work best for you, you’ll need to prepare the surface. Whether you’re jointing and fill skimming new plasterboard or looking to cover an old surface, you need to remember the five golden rules of surface preparation; to make sure it’s hard, cohesive, clean, sound and dry before you start. Hardness ensures that the filler won’t detach from the surface when applied. Scratch the plaster with your fingernail. If you easily penetrate it, the surface is too soft. You should also smooth over the substrate with your hand. If any residue comes off, it’s either too crumbly or powdery. Wall hardening products can assist in toughening the surface before covering.

A less cohesive substrate runs the risk of breaking away under the weight of the filler. Scrape away loose pieces first and rake out and dust any cracks or holes. To clean the surface, simply wash the wall or ceiling with a suitable product and rinse well. Stains or deposits can lead to blistering and separation, and the same effect can occur if the substrate is not dry. I recommend an anti-damp treatment or, on a raw substrate, even an anti-damp render filler. Similarly, fungicidal products should be used where there is mould or spores present on the substrate. This will ensure the surface is kept sound.

With your surface prepared, you’re ready to start covering the wall. If you’re jointing new plasterboards, you’ll first need to apply and smooth the filler all along the recessed edges between the boards. With a filling knife, use plenty of filler to the cavity and do so gradually, applying from several directions so as not to create any air pockets. Then smooth in much the same way. Take your jointing tape, measure the length of the filled strip, and cut it to size before bedding it in by gently pressing it into the filler using a blade or trowel to eliminate any trapped air. My top tip is to always smooth from the middle out. Working from top to bottom will cause it to drag. Once complete, apply a thin coat of filler over the tape, feathering out beyond the first coat. You can do this straight away, without the need to wait for it to dry. You could use a self-adhesive mesh scrim tape instead of paper tape. This eliminates the need for the back layer of filler onto which you would bed your paper tape. Avoid using this for corners though, as you’ll find scrim tape does not easily fold.

One advantage of using filler to skim plasterboard instead of plaster is that you do not need to seal the substrate first. You may have heard of a PVA solution being used to first seal the substrate, but fillers have breathable properties which prevent moisture build-up, so there’s no need to use this method. In fact, doing so could prevent these properties from working.

When it comes to skim-filling the whole surface, you have a choice of tools. Most professionals use a blade or trowel. If you don’t feel confident with that, there are now skim-filler plastering kits that come with the tools for the job and step-by-step instructions to skimming the entire wall or ceiling – first, by applying the filler onto the surface with a roller, and then smoothing it out with a specially provided blade.

A filler skimmed wall or ceiling will dry quicker than traditional plaster, meaning you can paint it all the sooner. Depending on the condition of your original surface, you may want to apply a second layer. Use high-grade paper to sand down to remove any imperfections. The grade of sandpaper will depend on the filler used but higher than 180 is a good guide. Anything coarser, and you’ll simply be scratching off all your hard work. Sanding filler can be dusty work, so I recommend using an electric sander with a dust extraction system.

Before painting over traditional plaster, a ‘mist coat’ of diluted emulsion is usually recommended to ‘seal’ the plaster because it’s very porous. There’s no need to seal a filler-skimmed wall before painting – it can be directly overcoated with a variety of coatings, although it’s always advised to check the technical specifications of the filler you’ve used.

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