On Wednesday, October 3, 2012 4:41:46 PM UTC-7, Existential Angst wrote:

Looking to patch up my stone walls, between stones. Was at
HD today, must be a half-dozen or more types of mortar -- or,
iiuc, "sandmix". Is there any type/brand particularly good
for patching stone walls? Any particular prep? I figger I'll
clean out the patch areas with a garden hose, etc. How to get
the stuff on/in the vertical wall face is the next challenge.....
Any tips? Along these lines (and of a previous post), I bought
two different types of grout, sanded and unsanded. Sanded is
like a fine mortar (iiuc), and unsanded is, I presume, very
fine particles that don't quite qualify as sand. The former is
for gaps 1/8 to 1/2", the latter up to 1/8". So that should
cover various slate-type, step repairs, with fine-ish cracks.

Mortar is usually sold in grades M (strongest), S, and N (weakest), and building walls generally need type S. It's not a good idea to use type M because it's stronger than the bricks. If you see pre-mixed mortar with no grade letter on it, assume it's type N, which is not up to code for buildings or high walls. I ended up going to a masonry supply because the person at the home center (not Home Depot or Lowe's) didn't know anything and claimed type N was the strongest mortar. The masonry supply charged the same as the home center.

I found it was easier to fill the joints from a grout bag (like a cake decorating bag, only much bigger and made of naugahyde vinyl upholstery) rather than pushing it in with a trowel from a hawk, especially vertical joints. Most grout bags are sold with a steel nozzle, but its opening is so small the mortar won't flow out, so I use the bag without it. Finish the joints the normal way, with a striker or trowel tip.