Health and Safety Info:
It’s advisable to wear gloves as lime dries your skin.
There are different types of lime mortar, but it is best to use a feeble mortar for internal works. Hot mix lime mortar is 100% breathable is the best suited.
The sand in the mortar will be a sharp sand, I generally use Cardewmires or Nosterfield. The Cardewmires sand will let you put the plaster on in 10mm layers, whilst the Nosterfield will be put on at 7-8 mm layers. The size of the largest aggregate in the sand determines the thickness you can do in one layer. Thicker scratch coats have the addition of hair or fibres.
The mortar wants to be knocked up by running it back into a mixer, this will bring it back to life and normally only takes a few minutes. A small amount of water can be added if needed.
For external rendering it is recommended to add a pozzolan to the mortar mix, this would be added @5-10% by volume. As a rule roughly 25kg of pozzolan to 1000kg of mortar.
If the property is listed the mortar should have animal hair added to the scratch coat, this can be goat, horse or cow hair, predominantly horse and goat hair is used. If the property is not listed a man made fibre can be used, I prefer the fibrin fibres which disperse very well.
Adding animal hair is not an easy or quick process, the goat hair comes in bundles normally about 100mm long. These need to be cut down into shorter lengths then added to the mortar, this can be done in different ways depending on what type of mixer you have.
Belle mixers are not good for this job as the hair seems to cling to the paddles, baron mixers are much better. I have seen some people add the hair in a wheel barrow and use a shovel to disperse the hair, the hair needs to be no more than 25mm long. The fibres can be added once the mortar is the correct consistency. To check if you have enough hair/fibres in your mix, put a trowel in the mortar, tap the trowel and you want to see a beard all the way round the trowel.
Wet the wall down before applying the mortar, if the wall is not level it will need to be dubbed out. This should be done with mortar and little galleting stones if required, hair can be added to the dubbing out mortar too.
Leave the dubbing out in an open pore state. This means don’t close it with a steel trowel effect, this reduces its breathability. Then mist the walls after it’s been finished, this will help slow down the carbonation and reduce the chance of shrinkage. This should also be done the following day.
If the mortar is too wet will cause more shrinkage, if it is too dry you will end up with a very sore arm and shoulder, try to get the right balance!!
Once the dubbing out is firm enough to take the scratch coat it can be applied, therefore damp the wall before applying the plaster. The plaster should be applied with a steel float, then when firm go over it with a plastic/wood float to open the pores ready to scratch it up.
I have seen jobs fail due to scratching up with that’s used for cement plastering. The opening up and scratching should be done when the plaster has stiffened, this could be 2 hours later but this depends on temperature and humidity.
I think the best tool for this is a wooden lath, the scratch should be done in diamond shapes approximately every 100mm. The scratch should be approximately 4mm deep and 4mm wide, this is done so the aggregate in the sand can fit into the scratch and the water will not trap in the scratch which is what happens if the scratch is done in horizontal lines.
Once finished mist the plaster a few times that day and day/days following, if you see signs of shrinkage push them in with your finger. Keeping the mortar down to 10mm and having enough hair/fibres in the plaster will prevent shrinkage along with damping the wall before work commences.
The next layer is identical to above barring hair/fibres and the scratch, this can be applied once the scratch coat is “green hard”. Drying times can vary due to conditions, but typically after approx 3-5 days the float coat can be applied and finished with a plastic/wood float. It is not recommended to undertake external works in the winter months when temperatures are low, care must be taken to prevent damage from frost.
Remember to use a breathable paint like limewash or Keim paint.