Q: I have heard that mortar joints should be tooled when they are "thumbprint hard."
What does this statement mean? And what is the problem with tooling the joint before it is thumbprint hard?
A: Mortar is thumbprint hard when the image of your thumbprint can be left in the mortar. When the mortar is too wet, it will not form a thumbprint. It just sticks to the thumb.
Mortar must be relatively stiff to be thumbprint hard. In this state the mortar compacts easily and provides proper compression against the brick units. Tooling the joint while the mortar is still very plastic does not provide the same degree of compaction.
Proper compaction is needed to provide good resistance to water penetration. It is often easy to see which joints were tooled early and which were tooled when they were thumbprint hard.
Joints that were tooled early often have multiple ridges on the surface of the mortar parallel to the direction of the tool. Ridges form as the wet mortar sticks to the tool. Joints are not properly compacted if tooled when they are too stiff. These joints must be aggressively worked and often have a darker appearance in the completed wall.
Joints should be tooled when they are just stiff enough to support a thumbprint for best compaction.