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Rendering Over Existing Render

Can you render over existing render is a question we have been asked frequently. Father Time is unstoppable and undefeated, therefore, inevitably, your rendered surface will suffer and degrade. Silicone Render offers more durability and longevity (up to 25 years). However, many other forms of render, or incorrectly applied silicone render can degrade. Cracks appear and render peels off, leaving your facade looking unsightly and system performance compromised. So what can you do to fix this?

Why does render fail?

Rendering refers to the application of a top layer or finish coat over a base coat in the context of plastering or rendering walls. The top coat is designed to provide a smoother finish, protect the base layer, and sometimes provide a decorative or textured finish. However, there are several reasons why top coat rendering might fail.

Inadequate Surface Preparation

If the base layer or substrate is not adequately cleaned and prepared, the top coat may not adhere properly. Dust, dirt, or loose material acts as a barrier.

Incompatibility of Materials

Different types of render materials might not be compatible. For instance, applying a cement-based topcoat over an acrylic basecoat might lead to failure.

Improper Mixing

If the render mixture isn’t prepared correctly with the right proportions of water, binders, and other ingredients, it can fail.

Application Conditions

Rendering in extreme temperatures (either too hot or too cold) can affect the curing process. If it’s too hot, the render may dry too quickly and lead to cracking. If it’s too cold, it might not be set properly.

Moisture Issues

If the basecoat isn’t fully dried before the top coat is applied, trapped moisture can lead to bubbling, blistering, or delamination. On the other hand, if the top coat dries too quickly due to direct sunlight or wind, this can also lead to problems like cracking.

Thickness of Application

Applying the top coat too thick or too thin can lead to issues. A thick application can cause the render to sag or crack, while a very thin layer might not provide adequate coverage or protection.

Poor Workmanship

Proper technique is essential when applying the top coat. Ensuring an even spread and avoiding trowel marks can make a difference in the outcome.

Structural Movement

If there’s any movement in the underlying structure, it can lead to cracks in the render.

Chemical Reaction

Sometimes, contaminants in the basecoat or on the substrate can react with the top coat, leading to discolouration or bond failure.

Ageing and Natural Wear

Over time, environmental factors like UV radiation, rain, and temperature fluctuations can degrade the render.

How to render over cracked render

Rendering over cracked render is a common task in building maintenance and refurbishment. If the cracks are merely cosmetic and not symptomatic of a more significant structural problem, the process can be reasonably simple.

Inspection

Before you start, inspect the cracked render to understand the extent and nature of the damage. Minor cracks can be treated with a filler, while larger cracks or areas where the render has detached from the wall will require a more comprehensive approach. Use a stiff brush to remove any loose material from the cracks and around them. This step ensures a good bond when applying a new product.

Filling

Use a scraper or a chisel to slightly widen the crack. This counterintuitive step allows for better penetration of the filler or repair mortar. Remove all dust from the crack using a brush or vacuum. This step ensures a clean surface for the filler to adhere to. For minor cracks, use a suitable exterior filler or masonry repair mortar. Push the filler deep into the crack using a filling knife or scraper, ensuring it’s well-packed. Smooth the surface to make it flush with the surrounding render.

Finishing

If larger areas of the wall are affected, significant areas of damaged or hollow render, it’s best to remove them. Apply a bonding agent to the existing sound render to help the new render adhere properly. Apply a new basecoat of render over the damaged area, ensuring it’s level with the existing render. If needed, apply a mesh or fibreglass matting to provide additional strength. Once the basecoat is set, apply the top coat or finishing render.

Allow the filled or rendered area to dry/cure according to the manufacturer’s recommendations. Once the repair is fully cured, you can paint or finish it to match the surrounding area. It’s often a good idea to repaint the entire wall to ensure colour uniformity, especially if the existing paint or finish has faded over time. If cracks were caused by movement, consider applying a flexible, paintable exterior masonry sealant in the cracks before repainting. This sealant can accommodate minor movements without cracking.

Which existing render is more prone to fail?

Various types of render are used in construction, and each has its advantages and disadvantages. Some renders might be more prone to failing than others depending on environmental conditions, application methods, and other factors.

  1. Traditional Lime Render:
    • Prone to: Frost damage if not adequately cured, erosion over time if not protected, biological growth.
    • Why: Lime render is breathable and therefore can absorb water. If the water inside the render freezes, it can lead to frost damage. Lime also tends to erode over time, especially if directly exposed to rain without a good quality finish or paint.
  2. Cement Render:
    • Prone to: Cracking due to shrinkage, low flexibility, and potential for efflorescence.
    • Why: Cement render is less flexible than other types, making it more susceptible to cracking from structural movement or thermal expansion. Additionally, cement-based products can show efflorescence, where soluble salts migrate to the surface and crystallise.
  3. Acrylic Render:
    • Prone to: UV degradation, staining, and potential for delamination from the substrate.
    • Why: While acrylic render is flexible and applied thin, it is sensitive to UV radiation which degrades the material over time. Being a thin coat, its bond to the substrate is crucial, and any compromise can lead to delamination.
  4. Polymer Render:
    • Prone to: UV degradation, and potential bond failure if not adhered properly.
    • Why: Polymer renders often contain a mix of organic binders that can degrade over time due to UV exposure. Proper substrate preparation is crucial for ensuring long-term adhesion.
  5. Insulated Rendering Systems:
    • Prone to: Delamination, potential for moisture buildup behind the insulation.
    • Why: These systems combine insulation with render, and improper installation can lead to gaps or moisture build-up behind the insulation. Moisture trapped behind can lead to mould growth or compromise the bond to the substrate.
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2 thoughts on “Rendering Over Existing Render

    1. Hi John. Silicone Render is least likely to fail due to its composition; it’s weatherproof, UV resistant, and can prevent biological growth, all of which can compromise a render system.

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