How to Install a Mortar Scratch Coat

Preparing the Substrate

• 1

  Rub a wire brush over the wall to remove paints and primers.

• 2

  Wash the wall with a de-greasing detergent and a scrub brush to strip off oils, grease and dirt, which may interfere with the mortar bond.

• 3

  Stretch wire mesh across the substrate. Drive galvanized nails through the mesh every 4 inches around the perimeter and every 6 inches through the field to secure the wire mesh to the substrate.

Mixing the Mortar

• 4

  Pour Type S dry mortar mix into a wheelbarrow, trough or large bucket.

• 5

  Create a hole in the center of the mortar mix with a hoe.

• 6

  Add water to the hole according to the mortar mix manufacturer's directions. If the outside weather conditions are humid, use slightly less water than called for in the directions. If the outside weather conditions are dry, add slightly more water than the manufacturer's directions.

• 7

  Pull the hoe from the dry mortar mix and put it into the water-filled hole, pulling it from all sides of the hole. Continue to pull the hoe through the mortar to blend the mortar to smooth, heavy consistency. If the mortar does not fall freely from the hoe, add more water and mix the mortar with the hoe. If the mortar does not stand in peaks and runs, add more dry mix and mix with the hoe.

Applying the Scratch Coat

• 8

  Mist masonry surfaces lightly with plain water. Dry masonry surfaces will pull the moisture out of the mortar, which will cause the scratch coat to become dry and crumble.

• 9

  Dip a trowel into the wet mortar mix and pick up half a trowel-full of mortar mix.

• 10

  Hold the trowel at a 45-degree angle to the wire mesh and press the mortar into the wire mesh while spreading a paper-thin layer over the surface. Continue to pick up mortar on the trowel and spread a paper-thin layer until the scratch coat covers all of the wire mesh.