Prep And Materials:

To patch you must have a container of fresh patching material (with no lumps in it), a bottle of white acrylic liquid bonding agent, some mixing containers and possibly some grey cement.

Example of typical corner damage shown here.

Step 1:

Mix a handful of the patching material with  just enough water as required to make a paste like mix.

Step 2:

Add a tablespoon of acrylic to use as a wetting agent, but not as a substitute for the water.

Step 3:

The repair mix should be as dry as possible, with just enough water so that it will be tightly compacted when forced in place.

Step 4:

Mix with a trowel or wooden tool for five (5) minutes until it has reached the stiffest consistency that will allow patching.

Step 5:

The color of the material should approximate the color of the wet stone.  If the material is too light in color, it may be too wet.

If darkening of the patching material is still required, add a pinch of the grey cement and manipulate with a trowel.

Step 6:

Thoroughly drench the damaged and surrounding areas of the stone with cool water and avoid doing the repair under hot sunshine. Apply the acrylic to the damaged area of the stone and allow to stand for five (5) minutes.

Step 7:

Shape the patch to the proper contour or squared edge of the stone.

Note: Metal tools should not be used to apply the material because the metal will sometimes change the color of the patch or draw excess cement to the finish.

Step 8:

After initial set, blend the patching material into the adjacent area with a wooden wedge or a piece of blue Styrofoam

Shown here is the final contouring of the patch using a wooden wedge.

Always try a test patch and obtain approval before proceeding with the balance of the work.

Shown here is a specially cut wooden corner tool which makes it easier to tool the patch at edge conditions. 

Alternatively, rub a piece of Styrofoam against the stone until it forms into the desired shape.

Do not use the material after it has started to set. (Approximately 20 minutes)

Step 9:

Cover the repair with a damp rag to aid curing.  Larger patches should be also covered with a plastic sheet.

After the patch has hardened, rub with a piece of #40 grit sandpaper to match the stone finish or use emery cloth if further darkening is required.

For best results, be sure patch has cured a minimum of five (5) days before final washdown with a masonry cleaner.

Some finishes which contain coloring pigments may require a mild acid wash during cleaning in order to obtain a better color match.

Over a period of months, curing, weathering and ultraviolet bleaching will have a tendency to further blend the finished color of the repair.