
David Halliday
Hi John, I am unsure as to what kind of material your wall is built with as you do not say. As a general rule the old friable mortar should be picked out by hand to twice the width of joint I.e 10 mm joint pick out 20 mm, do not allow anybody near your walls with a grinder as this will tend to not only cut out the old mortar but also cut into the walling material. As a general rule a bricklayer for brickwork a stonemason for stonework. Good luck with your project, Dave