Rendered exterior walls can look fantastic when they’re newly done, but sadly they can start to look a bit sorry for themselves all too quickly. As illustrated in the above photo, dirt, dust, pollution, bird droppings, algae and lichen can make fresh render look dirty, and cracks can start to appear.
Although the cracks can be a sign of unwanted movement in the building, often they’re due to weather damage (more on this shortly). Therefore, they are not a cause for undue alarm; it’s good to get these minor cosmetic issues sorted out promptly, but they’re not usually something to be up all night worrying about.
Dirt, dust, algae etc should all be cleaned off prior to repainting. Cracks and other damage to render should be filled – again, prior to painting – and that’s what we’re going to look at in this blog; we’ll look at the tools and materials you will need, and a step-by-step process for making repairs to render.
What Causes Damage to Rendered Walls?
Damage to render can occur for many reasons. Physical wear and tear can be an issue, particularly at ground level; one building that I work in regularly has an access ramp from the car park to the front door. The corner of the adjacent wall is forever being bashed by wheelchairs, push chairs, people carrying or wheeling large, heavy objects into the building, and even poor parking.
In less “high traffic” areas, hairline cracks are the most frequently occurring type of damage. Cracks usually appear where the render is placed under stress. At one extreme, this could be a sign of subsidence, but usually it is caused by new buildings “settling”, vibrations caused by heavy traffic (if near a busy road), or just the age of the building. Over time, the render can degrade and become prone to cracking and even crumbling.
Weather-related damage is possibly the most common cause – rainwater seeps into tiny cracks in the render, expands when it freezes, thus making the crack larger (so more water can get in, and so the cycle of damage continues). Even without water, extremes of temperature will cause the materials in the building to expand and contract slightly, which puts stress on the render and can cause it to crack.
Whatever the cause, it’s important to make repairs in a timely manner, to prevent rainwater from getting in and causing further damage.
Tools and Materials you will need for Repairing Render
- Old knife, chisel and/or scraper, or angle grinder (tools for scraping out and enlarging cracks in the render); if using an angle grinder, a small battery-operated one will be more manoeuvrable than a larger mains voltage machine.
- PPE – disposable gloves, mask and ear defenders (if using angle grinder).
- Old paint brush, water spray bottle, bucket of water, old sponge.
- Render or mortar mix, or suitable exterior filler (depending on the size of the repair – more to follow).
- Filling knife or trowel (again, depending on the size of the repair).
- Paintbrush.
Step By Step Guide to Repairing Render
Carve out the crack
Using an old knife, chisel, scraper or angle grinder (depending on the size of the cracks and how many you need to do), scrape out and widen the crack; the aim is to remove any loose material and make the crack wide enough for the filler/mortar to be pushed into. It’s very hard to force filler deep into a tiny hairline crack, but much easier with a wider crack.
Remove dust
Using an old paint brush, brush out any dust or loose material from the crack. You can use a proper dusting brush for this, but I prefer to keep mine “pristine” for indoor work. Alternatively, you could use a vacuum, if you have an electricity source nearby and the vacuum will reach to where you’re working.
Wet the area
Dampen the area using a water spray bottle – this will help the filler or mortar to adhere better.
Fill
Mix sufficient mortar or filler for your repair. Apply with a filling knife or small trowel and force the filler as deep into the cracks as it will go. Use a combination of an old brush and damp sponge to remove any excess filler, “feather” the edges and try to recreate the texture of the render.
This is often a bit of trial and error until you work out a technique that works. If you don’t get the texture right, you can sand the filler smooth once it’s dry, but smooth repairs are often quite visible when the surrounding render is textured. For example, I’m pretty sure the crack in the picture above has been repaired previously, and then sanded smooth. Unfortunately, the crack reappeared.
For larger repairs, you may need to apply a “base layer” of mortar first, to get much of the hole filled. Allow this to harden and then return to apply a “topcoat” which you can shape and texture as needed.
Allow to dry
Allow the filler to harder/cure thoroughly. NB if you are using cement-based mortar, you should leave it roughly 1 day per 1mm of depth, to allow it to cure properly before painting.
Stabilise
If your filler is powdery, you will need to apply stabilising solution to any repairs you have made, and any areas of chalky or flaky paint (scrape away loose material first), to help even out the porosity and create a more uniform and stable base for the paint to adhere to.
Now you’re ready to paint!
The Best Products to Use
OK, I’ll be honest, I haven’t scientifically tested every exterior filler on the market, but these are the two that I use most often, and they’ve never let me down. I’m pretty sure a lot of other professional decorators use them too, based on this article: Best Masonry Filler on the Market – Decorator’s forum UK.
Best Filler for Small Repairs
For small repairs and cracks, I like to use Toupret Masonry Repair Filler (available online here). I find it’s easy to mix and apply, you can fill quite deep cracks with it, and although it sets very hard, if you need to sand it you can (although you may then end up with a repair that is smoother than the surrounding masonry/render).
The only downside is that they recommend you don’t paint over it for 48 hours minimum, but that’s not necessarily a problem if you know about it in advance and can plan your work accordingly.
Best Filler for Large Repairs
For larger repairs (like the corner of the building I mentioned earlier which is always being bashed by something or someone!) I love Toupret Rock Solid Repair Filler (available online here).
It’s easy to shape and you can fill repairs of almost any depth in one go, as it doesn’t really shrink much. Being a powder, you can mix it to your own desired consistency, so for larger repairs make a stiff mix (not too much water), and it is less likely to slump. It does set rock solid, so do any shaping, smoothing or texturing before it goes off – your chances of being able to sand it the next day are not good!
FAQs
What filler to use on render?
Toupret Rock Solid filler and Masonry Repair filler are widely regarded as the best fillers to use to repair render.
Can I paint over hairline cracks in render?
There is a product called Bedec Extra-Flex that can be used for painting over hairline cracks on exterior render. The paint floods and fills the cracks, then acts as a binder, preventing the cracks from reoccurring.
Should I worry about cracks in render?
You only need to worry about structural cracks. Luckily, most cracks I’ve come across as a professional decorator have been superficial, so easily repaired. You can check; structural cracks will be in the block or brick under the render, whereas superficial cracks will only be in the render itself. The other thing you might need to worry about is whether the render has ‘blown’, which basically means it’s coming away from the wall underneath. To test, just tap on the render. If it sounds hollow, you may have a problem.
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