What is a spatterdash coat?

A spatterdash coat is a layer of mortar that is dashed or sprayed onto a background, by hand or by machine, so that the next coat bonds better. It forms a rough “key” into which the following layer can grip. That next layer may be a coat of plaster or render, but also tiles bedded in mortar. In Dutch the technique is called spritsen, borrowed from the German spritzen (to spatter); in German the coat is called Spritzbewurf. A spatterdash coat is a background preparation rather than a full render coat in its own right.

Place in the plaster build-up

1. Structural background (masonry, concrete)
2. Plaster base (the background itself can act as the base)
3. Spatterdash coat, if needed
4. Scratch coat, if needed
5. Plaster or render coat
6. Finish, such as limewash, paint or wallpaper

Two ways of applying it

• Partial, net-like. The mortar is dashed on so that about half of the surface is covered. This improves adhesion on smooth, low-suction backgrounds such as concrete.
• Full coverage. A closed coat evens out uneven or high suction, for example on mixed masonry of different brick types.

Application

1. Clean the background. Remove dust, loose material and release oil. A spatterdash coat will not bond to dirt.
2. Leave it rough. Throw the mortar on firmly and do not smooth it. The rough texture is what provides the key.
3. Allow the waiting time. Let the coat harden before applying the next one. Manufacturers often state at least one day; follow the data sheet.
4. Keep it damp in hot weather. Drying out too fast weakens the bond.
5. Use a matching system. Preferably use a spatterdash and render designed to work together.

Common mistake: rendering straight onto smooth concrete without a spatterdash coat or bonding agent. The render seems firmly attached at first, but later sounds hollow or comes loose.

Related terms

• Scratch coat
• Plaster coat
• Plaster base
• Rough rendering
• Rendering a first coat
• Facade rendering
• Lime mortar

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.