Lime bloom (or efflorescence) is a natural phenomenon, and while it’s unsightly, it’s nothing to worry about structurally, and is no indication that your render has been compromised or improperly applied.
Lime bloom appears as a general lightening or discolouration on coloured cement-based products that are otherwise sound and fully cured. It doesn’t result from a fault with the product or the workmanship but rather from the intolerance of cement to certain conditions during its early stage of hydration. It can occur when rendered elevations are subjected to damp conditions, and/or low temperatures during early curing. The resulting thin film of what is essentially lime on the surface of the product may be a slight eyesore but certainly doesn’t affect the integrity of the render.
So how can you get rid of it?
If it is indeed lime bloom it will weather away itself naturally over time provided the building is exposed to the elements. One way to check it is in fact lime bloom that you’re dealing with is to dampen the surface with water. If it disappears, and reappears when dry, it is more than likely lime bloom.
If you don’t fancy waiting for it to weather away, you can speed up the process by washing the affected area with a mild hydrochloric acid cleaner. Weber unfortunately doesn't manufacture such a product, but we suggest that you use a product with a strength of between 16% and 20%. Before applying to a large area, the wash should be patch tested on an inconspicuous spot to make sure it’s suitable. Start with a dilution of 1-part clean water to 1-part hydrochloric solution and increase the strength if need be depending on the severity of the lime bloom.
Dampen down or soak the area to be treated with clean water to avoid the acid cleaner being drawn into the surface of the render.
Use a non-wire brush to apply the cleaner over the whole of the affected area. If you’re working with a vertical surface, work from the bottom up.
Allow the cleaner to work for 10 minutes.
Thoroughly rinse off with water, using a jet washer on the fan setting, at a strength of no higher than 60 bar, working from the top down.
Soft white bristle nylon brushes are best for application. Do not use metal containers. When using this product, protect surrounding areas as it can be corrosive to a range of metals including aluminium, chrome, copper, stainless steel and galvanised enamel. It may also mark materials such as paint, leather and laminates.
In certain cases, you may need to apply more than one treatment to completely remove the lime bloom. Always treat an entire elevation and not just an isolated area.
When using acid based cleaners, it’s important to follow the health and safety advice provided by the manufacturer.
At Weber, we’ll be more than happy to advise you on how best to deal with lime bloom, or a range of other issues you may encounter. You can find the contact details for our customer service and technical advice teams on our contact page.