rwa.martin
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I'd be grateful for advice on the best mix for harling. I intend to use nhl 3.5, 2:1 ratio with builders sand. What volume of 6mm down aggregate should I add to get a suitable harling mix?
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Recommended Harling Mix
- Thread starter rwa.martin
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Gervase
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- North London
The Scottish 'harling' can use an aggregate that is very similar to a coarse ordinary sharp sand, with a grain size of 6mm down to fines. This is mixed to a ratio of around 1 part NHL5 hydraulic lime to 2.5 parts aggregate, mixed very sloppy and applied in two or three coats (the first coat can be pressed down with a wooden float if you want).
The success comes down to the consistency and technique. You want a harling mix to be fairly sloppy; think of porridge - runny enough to flick off the trowel, but not so watery as to have all the aggregate fall off. You also want coverage to be uniform, so you need to get the 'flick' right - your wrists and elbows will probably ache like hell afterwards!
The English and Welsh equivalent is roughcast or wet dash, and is often made with a fat-lime (non-hydraulic) mix of 1:2 lime:sand to make the 'carrier' and a 6-8mm gravel aggregate added to give a coarse but sloppy slurry mix. For exposed surfaces you can use NHL3.5 in place of fat lime. The finish is dashed onto a well-scratched lime surface and brushed down with a stock-brush dipped into more of the slurry to give a uniform texture. Again, uniformity of flick is important, but the brushwork can mask minor inconsistencies.
For both treatments, have some poly-tarpaulins at the base of the wall so you can scrape up and re-use the stuff that falls to the ground. It's the sort of job where it really helps to have three people; one mixing, one wetting the wall and scraping up the excess and one flicking. And it's messy, so wear goggles!
The success comes down to the consistency and technique. You want a harling mix to be fairly sloppy; think of porridge - runny enough to flick off the trowel, but not so watery as to have all the aggregate fall off. You also want coverage to be uniform, so you need to get the 'flick' right - your wrists and elbows will probably ache like hell afterwards!
The English and Welsh equivalent is roughcast or wet dash, and is often made with a fat-lime (non-hydraulic) mix of 1:2 lime:sand to make the 'carrier' and a 6-8mm gravel aggregate added to give a coarse but sloppy slurry mix. For exposed surfaces you can use NHL3.5 in place of fat lime. The finish is dashed onto a well-scratched lime surface and brushed down with a stock-brush dipped into more of the slurry to give a uniform texture. Again, uniformity of flick is important, but the brushwork can mask minor inconsistencies.
For both treatments, have some poly-tarpaulins at the base of the wall so you can scrape up and re-use the stuff that falls to the ground. It's the sort of job where it really helps to have three people; one mixing, one wetting the wall and scraping up the excess and one flicking. And it's messy, so wear goggles!