I recently bought a house built around 1930 or a little before. The foundation is "field stone" (specifically, the home inspector told me it's Wissahickon Schist). It looks like the inside walls were originally "parged" with some kind of soft, brownish mortar that now crumbles easily. About 2/3 of the walls have been recoated sometime later, with a hard Portland cement mix. Some areas of that are now cracking and falling off, basically like the candy shell of an M&M, revealing the softer stuff underneath.
I've read that using Portland cement for parging a stone foundation is a no-no because it traps water in the foundation. However, given that most of it is already done that way, and that job is probably a few decades old, should I just patch it up, or should I chisel off all the way down to the stone faces and start over with a softer mortar mix? That is doable, it would just take time and generate a lot of waste masonry that I would have to pay to get rid of.
Before I do any of that, I'm planning to re-point from the outside on the exposed part of the foundation, because I do have a few water leaks coming through the walls in heavy rains. So, this is just early research so I can start to figure out a series of steps to take to get this basement done over the next few months. The basement is going to remain unfinished, so I don't need it to look beautiful or to be absolutely 100% watertight, but I just want to smooth out the walls and stop the water as much as possible, without doing something that will cause the foundation stones to deteriorate over the years.