I have very often made my own carving tools, I have renovated listed buildings and buildings of historic interest for many years. It also included the repair of sandstone, bathstone etc and included architectural decorative stone (corniches, plinths, flying buttresses etc and even carved render into block on a large hotel about 20yrs ago using the same method. If you cut it out and it is a bit to wet have a small wet artists brush and rub it along the joint and it will smooth it out, if it is to dry and is grainy mix a cement/water grout and do the same method. I might not be the best plasterer about, but nobody can tell me how to repair or carve stone or s/c render. it's really simple them methods will work every time.