Facade treatment: lime, silicate, and weatherproof paint

Outdoor painting work sets different requirements than indoor work. A facade must withstand rain frost sun and UV light year after year and the choice of product determines whether the surface holds and breathes correctly. As a building painter you must know the most important facade products and understand when one is better than another.

§Lime and limewash — the oldest choice

Liming is one of the oldest forms of façade treatment and is still used, especially on older and listed buildings. Limewash is based on slaked lime (calcium hydroxide), which absorbs CO2 from the air and hardens by carbonation, becoming the same material as many old walls are made of. This provides a highly vapor-open surface that allows moisture to escape from the wall, but limewash is also more sensitive to weather during curing and generally has a shorter lifespan than modern products.

§Silicate paint — chemically bonded to the substrate

Silicate paint is based on potassium silicate (water glass) and reacts chemically with mineral substrates such as plaster concrete and brick in a process called silicification. The result is a surface that becomes part of the substrate instead of just sitting on top of it as a film. Silicate paint is very diffusion-open and generally has good durability but requires a clean mineral substrate to bond properly — it does not work well on organic or already plastic-painted surfaces.

§Silicone and acrylic products

Silicone resin paint works on a principle often called the 'raincoat principle': the surface repels driving rain but is still open enough that moisture vapors can escape from inside. Acrylic paint is the most widespread and typically cheapest choice but is at the same time the most diffusion-resistant of the four main types — something that can become a problem if the substrate already has moisture problems or if multiple layers of acrylic are built up over the years.

§Standards and documentation

Outdoor coatings for masonry and concrete are covered by the European standard DS/EN 1062-1, which classifies products based on among other things diffusion openness and water absorption. Plaster and felt based on organic binders are similarly covered by DS/EN 15824. As a professional you don't need to know the standards by heart, but you should know they exist and that a product data sheet referring to them gives you an objective basis for comparing products—instead of relying only on the manufacturer's marketing.

§The weather determines when you can paint

Facade work is more weather-dependent than indoor tasks. Temperature, humidity, wind strength, and risk of rain or night frost determine whether a treatment can cure properly. Many facade products have a minimum application temperature and a period after application where they must not be exposed to rain. Planning facade work according to the weather forecast is therefore a fixed part of the task—not a convenience.

• 01Check product datasheet for minimum and maximum application temperature
• 02Avoid application in direct strong sun on hot surfaces.
• 03Plan machines, tools and sequence
• 04Be aware of wind conditions when spraying at height