Why is three coat vs one and a half coat plaster better?

[inwood]

Downstairs all my rooms were plastered with three coat work (scratch, devil, finish coat). Upstairs it's pretty much all 'coat and a half' (devil, finish).

As far as I can see the finish I'm getting on my first bit of coat and a half upstairs is identical to the devil coat I'd get on three coat work.

Does anyone know why three coat work is regarded as giving a better finish?

 

[Gervase]

Three coat gives you more chance to get an absolutely flat surface - and three-coat usually has a very fine float coat, whereas coat'n'arf (or cottage plaster) often has the finish coat made up of the same aggregate as the scratch, with just a bit more lime and no hair.
Thus three-coat was used for the posh 'public' rooms downstairs, while coat'n'arf was for the bedrooms, back rooms and staff areas.

 

[inwood]

Thanks Gervase. Could you advise on another issue I'm tackling at the moment?

The upstairs bathroom plaster has been repaired, patched, skimmed with gypsum, patched with bonding, hydrated lime finish coat and then painted in plastic paint, I could go on. Needless to say it's all a bit of a botch job. We've removed the gypsum, bonding and plastic paint but even the old lime patch repairs were appalling with ridges between the patch and the original of up to 10 mm.

Since there's so many patches, my thought was that I could apply another coat'n'arf over all the plaster to level the surface before applying the finish coat. A good idea?, if so would I need to apply a key for the new coat'n'arf? and how would that best be acheived given the largest aggregate size in the new coat would be about 4mm?

And would this now become four coat work (two 1.5 coats plus one finish)? :wink:

 

[Gervase]

Sounds like a right dog's dinner!
If the stuff you've got left is just bare lime-based plaster of varying quality, the best thing would be to scratch it roughly with a coarse wire brush or devil float (a home-made job made of a piece of wood with a few masonry nails hammered through is fine) - wear eye protection, as all sorts of stuff will fly around - then apply a tack coat of limewash over it and skim it with whatever you're using for a float coat. Unless, that is, the thickness difference is still more than a pencil's width. In that case, go for coat and a half.