I need to touch up my front porch, but which mortar is best? It was cast in place in the 1930s with a slip cast on top and roughcast on the sides, the steps are smooth rendered. Quite a bit has fallen off this past winter and out of some kind of ingrained habit I've bought C-mortar for these kinds of projects, but the other day I watched as a smooth-rendered foundation was repaired at work, a similar job, and they used B-mortar? What's the difference, and which is best? I'm knowledgeable about electricity, cars, mechanics, and airplanes, but mortar is a blank spot on the map.
One preferably wants to repair with a plaster that is weaker/softer than the substrate (or optimally the same). Since one often doesn't know exactly what the substrate is made of, c-mortar is usually the safest choice. However, b-mortar is harder and more suitable for a porch step, so go with that, use a 1mm fraction as it is easier to work with.