Mixture ratio: 1 part Portland Cement, 1 part lime, 6 parts aggregate.
These are from the specifications for a historic renovation I bid on. It is obvious that the people that write these have never put mortar on the wall before. The truth is, I could probably put on what I want to, but there is a chance that someone would try to enforce this mix.
1/4 part lime and 2 parts sand is what we use.
1. Portland Cement Mixes: Scratch Coat: For cementitious material, mix 1 part portland cement and 0 to 3/4 3/4 parts lime. Use 2-1/2 to 4 parts aggregate per part of cementitious material. Brown Coat: For cementitious material, mix 1 part portland cement and 0 to 3/4 parts lime. Use 3 to 5 parts aggregate per part of cementitious material, but not less than volume of aggregate used in scratch coat.
These specs are from a renovation in a major university. I hope no one follows these or this stucco will fall apart.
I have heard this expression a lot and I hate it. What this means is that a "poor" mix, or heavy on the sand has fewer shrinkage cracks. A poor mix tends to be weaker so there may be a trade off in quality, that is reducing the shrinkage cracks in the brown coat can weaken the finish coat. Additional sand will even stregthen the mortar on high suction surfaces like block. My formula is one 94 lb. bag of portland, 1/4 bag of lime and 20 shovels, or 4 cubic feet of sand. Buckets are a more precise method of measuring sand. A five gallon bucket is 5 shovelfuls and is right at 1 cubic foot. The reason I keep the mix the same scratch brown and finish, is for uniformity in expansion. The weak part of stucco when you hit it with a sledge hammer is the bond between the coats.
Someone sent me a link to an excellent article in the Journal of Light Construction about stucco, where a rich scratch coat and a poor brown coat was used by the contractors. Here's the article.
The main reason for a rich scratch coat, that is less sand, is for more suction, or absorbing the water out of the brown coat more rapidly. This firmness allows getting the wall in good shape and avoids sagging and bagging.
An example of a rich scratch coat would be 15-16 shovels per bag of portland, and a poor brown coat maybe 25-30 shovels. If the scratch coat is real rich the additional sand won't weaken the brown coat.
For conventional plaster, a rich scratch coat, for example 12-14 shovels per 100 lb. of plaster, can allow the ceiling to be browned the following day, instead of letting the ceiling set and dry for two days.
As a warning, if a very rich scratch coat is used for conventional plster, it should be doubled up, or browned the next day. We put in suspended lath nd plaster ceilings at the Pepco plant in Alexandria, Virginia years ago. I used 12 shovels per bag with the idea we would double the ceiling the following day. We didn't double the ceiling for two days and had way too much suction. I ended up soaking the ceiling with a hose to finish it.