Waaaa! Lime finish coat cracks

[Anonymous]

Possibly not so useful - it seems to suggest that it takes years to learn to plaster! Weeell -maybe - but people do manage in less time than that!


http://www.oldhousejournal.com/notebook/npsbriefs/brief21.shtml

 

[Anonymous]

Cracking often happens because the plaster is applied too wet - you want a far stiffer mix than with modern gypsum plaster, and you'll notice the difference when you apply it.
A well-mixed ime plaster won't 'float' across the wall but will need to be pushed, and pushed hard. You'll end the day with aches and pains and forearms like Popeye, but that's how lime plaster is meant to be!
Ad when it's on the wall, then you start beating it up with a wooden float, closing up the cracks and giving the surface a nice smooth finish. Remember to use a mist spray every few hours, and don't turn your back on the plaster - keep an eagle eye on it and buff with your wooden float when you see cracks appearing.
Depending on how thick you've applied it, the plaster will surrender after a day or so and lie there, uncrazed and smooth, and you'll be able to tell yourself you can lime plaster.
PS: Don't add anything calling itself a water repellant, or anything else really; you plaster should consist of nothing other than water, non-hydraulic lime and sand (or ground glass cullet), plus animal hair or plastic fibres for base and scratch coats.

 

[Anonymous]

Ah Gervase,
I do treasure your responses!
Again-you too say "non-hydraulic lime" meaning you yourself perhaps begin with the putty itself and no dry lime.
Can you describe specifically what you mean by "beating" the plaster once it's on the wall? How do you prevent it from adhering to the float and pulling off? And how large of a wooden float do you use? (We really truly have a paltry choice in plastering tools available in our home repair stores.)

 

[Anonymous]

Putty or dry - it's like Gulliver's Travels and the war between the big-endians and the little-endians. Reputations have been made and lost in the arguments between the two, and possibly blood spilled...

It doesn't really matter a hoot, however. In either case, the raw material is slaked lime - Calcium Hydroxide (CaOH) - with no silicates, magnesium compounds, gypsum, E numbers, sweeteners or additives whatever. Just plain, vanilla hydraulic lime.
Putty is nice because the water is already in it in roughly the right proportion, which makes getting the consistency right a doddle. Fresh, uncarbonated powder is nice because it's easier to knock up (in the UK sense!) in a bell mixer. Either will give you a perfectly serviceable plaster.

As for 'beating', it merely means working over the surface with a float. Wooden floats are better for this because they're slightly rough and don't have the 'suction' of a polished steel float, which will tend to bring all the lime to the surface when the plaster is wet and leave the lower levels of aggregate starved of lime.
Keeping the wooden surface of the float damp stops the plaster from sticking to it, and the 'angle of attack' to give a good smooth surface is something that you can only really pick up by doing it; ideally you're using the float in a sweeping motion on plaster that's just gone off the wet sloppy stage but which is still soft enough to mark with your fingertips, the leading edge maybe a millimetre higher than the trailing edge, and you're also pressing down quite hard to compact the plaster beneath the float.
As for size, it really is a matter of taste (no smutty comments, please!). Mine's 11x5in. but smaller ones are handy for finishing off down the sides of architraves and into corners. Cross-grained floats are more expensive, but better to work with because the grain lies across the direction in which the float is dragged. This means (a) it doesn't get grooves worn in it and (b) the surface stays flatter and more uniform. Even a seasoned beech cross-grained float will wear out, so if you find them hard to come by, order/make a couple at a time.



Types of float

 

[Anonymous]

Ever tried using less lime but adding subsoil in its place?