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Applying Render to Cement Board

Cement board provides a solid, durable surface that can withstand the extreme weathering effects of wind, rain and snow. The most common two types of cement board available on the market are Permabase (from National Gypsum) and Knauf’s Aquapanels. The boards tend to be made up of Portland cement, aggregate and fiberglass mesh. This combination provides an impact resistant barrier, but the real beauty of these cement boards is that they don’t rot or swell when they come in to contact with water, so they survive a long time even when located on the exterior of the property.

Cement boards can be finished in numerous different ways, but one of the most effective ways to finish them is to apply render to them. Applying render to cement boards provides a fantastic looking finish, but also offers a further level of protection to the boards. So how do you go about it?

Applying render to cement board

The first thing that you need to do is to prime the cement board. We recommend priming the cement board with EWI-310 Universal Primer, because the presence of silicate in the primer allows the basecoat layer to key into it – ensuring a really strong adhesion between the cement board and basecoat layer.

EWI-310 Universal Primer comes in 20kg buckets and each bucket can cover between 60 and 80m2 depending on absorption of the cement board. The primer is applied to the cement board by a brush or roller and is red in colour allowing you to see where on the wall it has been applied. We recommend allowing this to dry completely before applying the next layer.

Applying the basecoat to the cement board

Applying the basecoat to the cement board is relatively easy – we recommend using either EWI-220 or EWI-225. The EWI-225 is slightly stronger than the EWI-220 so it depends on how solid you need this façade to be. Both the EWI-220 Basecoat Adhesive and the EWI-225 Premium adhesive come as a dry mix and therefore need to be mixed with water prior to application. Once the adhesive has been mixed with water and the grey putty has formed, it needs to be applied to the cement board with a notched trowel. We recommend using a 10mm notched trowel, this is the best size of trowel to apply a 6mm layer of adhesive to the cement board.

Once the adhesive has been applied it is time to embed the fibreglass mesh within it; it is this mesh that gives our render systems such flexibility. EWI-640 Reinforcement mesh comes in 50m2 rolls (1mm wide), and this needs to be applied in vertical strips within the adhesive. The strips of mesh need to overlap one another by 10cm at edges and this is needs to be sunk within the layer of adhesive – this can be done by using the flat edge of the notched trowel and drawing this up from the bottom of the wall – this will pull the adhesive through the holes in the mesh.

Remember the render topcoat is incredibly thin so therefore the basecoat layer needs to be completely smooth before applying the render. If you have embedded the mesh and the finish isn’t completely smooth the easiest thing to do is produce a very wet adhesive mix and apply this to the basecoat for the first layer of adhesive goes off. This should allow you to create this smooth layer ready for application of the render.

Once the basecoat layer has completely gone off, you are ready to prime the wall ready for the application of the render top coat. The primer depends on the render used, but provided you match the primer with the render, then the principle is exactly the same. Take for example EWI-333, our Silicone Silicate Primer which comes in either 7kg or 21kg buckets. This primer is applied to the basecoat layer using a paintbrush or roller, with the 7kg bucket covering approximately 20m2 and the 21kg bucket cover 70m2 of wall.

Applying render to the cement board

It is time to apply the render to produce the finished façade! All the EWI-Pro renders are thin coat renders, therefore the thickness of the layer is determined by the size of the granulate in the render. For example EWI-040 Silicone Silicate render comes in 4 thicknesses, 1mm, 1.5mm 2mm and 3mm. The layer of the render should therefore be either 1mm, 1.5mm, 2mm or 3mm depending on the render used – it should be no thicker!

This is really important to remember, and will ensure the finish is good. We have heard of several reports of people applying the render to thick and not being able to achieve the required finish. With that in mind, apply the render to the cement board with a trowel and then pull off all the excess with the trowel. Once you have taken off any excess, leave the render for 5 mins and then using a plastic float, work the render in a circular motion to give the final finish.

There you have it – how to apply render to cement boards!

A recap of the materials required to render cement board

EWi-310 Universal Primer

EWI-220 Basecoat Adhesive / EWI-225 Premium Adhesive

EWI-640 Reinforcement Mesh

EWI-333 Silicone Silicate Primer

EWI-040 Silicone Silicate Render