Harling is a Scottish term describing an exterior building surfacing technique. The theory of harling is to produce a long-lasting weatherproof shield for a stone building. A pigment can be embedded in the harled material, thus obviating the need for repainting. Harling is a technique used to surface many Scottish castles, but it is also used for a variety of common everyday building types. It is long-lasting and practical and well suited to the Scottish weather.
Harling is a process of covering stonework, using a plastering process involving a slurry of small pebbles or fine chips of stone. After a wall is complete and has been pointed and allowed to cure then a base of lime render is applied to the bare stone. While this render is still wet a specially shaped trowel is used to throw the pebbles onto the lime surface, which are then lightly pressed into it. Harl, being mostly lime render, cures chemically rather than simply drying. After this setting process the harl is sometimes lime washed in various colours using traditional techniques.
Cement base render cannot not be used in place of the lime, as it forms an impermeable barrier to moisture. The traditional harling process allows water to escape. This means that if cracking occurs the water is trapped behind the surface and often penetrates inside the softer stone, thus causing the rapid deterioration the technique is designed to prevent. For similar reasons modern barrier paints should not be used in place of traditional lime washes.
The technique of harling is utilised in a large number of famous Scottish buildings including Crathes Castle, Craigievar Castle, Muchalls Castle, Myers Castle, and Stirling Castle.