Using a trowel is fine, but to do it properly with a trowel you are supposed to ‘throw’ the chaux into the joints…not scape it in. Which takes a lot of skill. I think you will find washing up gloves and using your hands ‘throwing it in’ is better and more accurate !!!
Filling the joints with a trowel is a complete waste of time.
Jointing old stone walls is an art.
Number of other points.
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No mortar…should be banned in old buildings.
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Always mix the chaux by hand and not with a mixer. They should be banned to.