I've got an old 1890's house in the midwest with lime mortar and soft bricks. The front (street) side of the house was done with a better quality brick and has held up with minimal repairs.

However, the other 3 sides, and especially the ones that face south and west, have a lot of mortar deterioration. You can see where there have been many attempts at re-pointing and repairs over the years, where it now looks like a patchwork. There are still holes in the lime mortar popping up. There are several houses built this same way on the block, and other neighbors have had problems with bugs/bats crawling in.

I'd really like to just parge over the bricks and mortar and previous repairs. But I've found conflicting info about mortar or parging with soft bricks and lime mortar.

Is there a lime/sand/portland mix ratio? Or lime/sand only? I've read that anything with portland cement will be too hard and too water resistant causes the bricks and remaining lime mortar to fail.

Do I need to use a bonding agent or just water? I really don't want to dig out all the existing mortar and re-point, so that's why I'm looking at parge options.