Poor Surface Preparation
Poor surface preparation is one of the most common spray rendering mistakes and it affects everything that follows. If dirt, algae, loose paint or old render are left on the wall, the new coat will struggle to bond. Over time it can blister, flake or fall away in patches. Many problems start because cracks are not opened up and repaired, or hollow sounding render is not removed.
Rushing this stage to save time is false economy. Careful cleaning, repairs and degreasing give the render a sound, stable base so it can stick properly and last longer. This means washing, biocide treatment, scraping, sanding and patch repairs should be planned into every job, not treated as optional extras from the start.
Skipping Primers and Key Coats
Spray rendering systems are designed to work as a complete package, and primers or key coats are a vital part of that package. A primer evens out suction in the substrate so the finish does not dry too fast in one area and too slowly in another. Without it you risk patchy colour, hairline cracking, debonding and staining that are hard to fix once the wall has cured. Primers also help block background staining and improve bond on smooth or low suction surfaces. Key coats or base coats often include fibres and special binders to bridge small cracks and provide extra grip for the final coat. Skipping them can lead to hollow spots, drummy areas and poor adhesion.
Always follow the full system, including primers and key coats recommended by the manufacturer, and apply them at the correct rate and thickness for each substrate type. A common mistake is using a general purpose sealer instead of the specified primer because it seems cheaper or is already in the van. This can save a small cost on the day but greatly increases the risk of failure later.
Another error is over diluting primers or base coats with water so they go further, which weakens the film and stops them doing their job. Rushing the work by spraying the finish coat before the primer or key coat has dried properly is just as risky. Taking time to use the right products in the right order gives a far more reliable result overall.
Wrong Mix or Product Choice
Using the wrong mix or render product is another frequent mistake in spray rendering. Each system is made for certain substrates, such as blockwork, brick, concrete or insulated boards. If you match them poorly, you can get cracking, crazing or damp problems later. Adding too much water to make the mix flow more easily also weakens the render and can cause colour changes and dusting.
On the other hand, a mix that is too stiff will not pump or spray correctly and may not bond well. Always follow data sheets carefully and stick to the right product for each job. If you are unsure, speak to the system supplier, who can confirm exactly which mix is suitable for your project.
Spraying in Bad Weather
Spraying in bad weather is one of the biggest causes of failed spray render, even on otherwise well planned jobs. Low temperatures slow down curing and can allow frost to damage the surface before it has fully set, leading to powdery, weak areas. Very high temperatures or strong sun cause rapid drying, which leads to shrinkage cracks, crazing and poor, patchy colour. Wind can blow dust and debris into fresh render and cause an uneven texture that is difficult to repair neatly. Driving rain during or shortly after spraying may wash out pigment, leave streaks and reduce strength in the face of the wall. Problems also occur when walls are in full shade and stay damp for long periods, as the render can take much longer to cure than expected. This is a particular risk on north facing elevations and narrow side passages with little air movement.
Good planning starts with checking the forecast and understanding how sun, shade and wind affect each elevation through the day. Protective sheeting, windbreaks and curing guidance from the manufacturer are there to help, but they cannot overcome very poor weather choices. If conditions fall outside the stated limits for temperature, wind and humidity, it is usually safer to delay work.
Keeping a simple site weather log and taking photographs during application can also help if problems arise later and you need to show what conditions were like. Training staff to recognise risky situations, such as sudden temperature drops or approaching showers, gives them confidence to pause work rather than push on and hope for the best later.
Incorrect Thickness and Coverage
Incorrect thickness and coverage during spray rendering quickly lead to problems. If the coat is applied too thinly, it may not cover the background properly, so you can see patterning in the finish, and it may not give enough protection from rain and frost. Where render is too thick, it becomes heavy and more likely to crack, slump or pull away from the wall.
Uneven passes with the spray gun create highs and lows that show through after ruling off. Always monitor material use, use depth gauges where needed and check thickness across the wall so the whole surface is even. Recording nozzle size, pressure settings and passes per square metre helps you repeat successful results on walls in future.
Our spray rendering service focuses on doing the job right first time. We clean and repair walls, use the correct primers and mixes, and plan work around the weather. Our team controls thickness and coverage on every surface to give an even, durable finish that looks smart and protects your home for years.
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