Repairing Sandstone

Repairing Sandstone

Leanne Constantino9/17/21

Sandstone has a range of colours and natural patterns that make it beautiful.  It is a popular choice for patios and terrace applications in outer spaces. Sandstone is a soft and porous material, which is why care and maintenance are so important.

Below is some brief practical guidance on repair of sandstone.

1) Cut Around Unsound Areas

Make 3/4-inch cuts around any unsound area on the sandstone sill with a wet diamond blade (refer Kaptive C&P Restoration & Preservation). Cutting a boundary around the compromised sandstone makes chisel removal more precise.

2) Remove Damaged Sandstone

Chip away the stone defined by the diamond cuts with a pneumatic hammer and chisel, or diamond disc. Tap the chisel head gently to remove damaged sandstone and to prevent trauma to the underlying sandstone base.

3) Create Mechanical Keys in the Sandstone Sill

Create "mechanical keys" in the sandstone sill. Keys, or small holes drilled into the stone base, should be 1/2 inch in depth and 1/2 inch in diameter. If you're repairing a large section of the windowsill, space the mechanical keys approximately 3 inches apart. Where sills are particularly deep, create two staggered rows of mechanical keys.

4) Wash the Sandstone

Wash the sandstone surface before applying the patch. Use water and a soft brush to remove sedimentation. If the sill is discoloured by pollution or fungal growth, add 1 part household bleach to 15 parts water. Use gentle strokes to prevent etching of the sandstone.

5) Create a Slurry Coat

Mix 1 part portland cement, 2 parts type lime, 6 parts sand and water to create the slurry coat.

6) Apply the Slurry

Spread a thin coat of the slurry layer with the trowel to the sandstone sill and work it into the surface. Concentrate this sandstone filler on cracks, mechanical keys and ridges in the stone.

7) Create the Scratch Coat

Mix 1 part white portland cement, 1 part type lime, 6 parts sand and water for the scratch coat (refer The Balance Small Business.)

8) Apply the Scratch Coat

Spread the scratch coat with a trowel onto the sandstone sill. Keep the sandstone repair resin to a depth not to exceed 3/8 inch. After applying the first layer, score in a criss-cross pattern with the trowel. Scoring the damp patch creates better adhesion for subsequent layers.

9) Set Your Timer for Two Hours

Wait two to four hours before applying the second, and all future scratch layers.

10) Create the Finish Coat

Combine 1 part white portland cement, 1 part type lime, 2 to 3 parts sand, 3 to 4 parts crushed stone, dry pigments -- natural or synthetic -- and water to use as a finish coat. Pigments and stones are added sandstone repair mortar as needed to match the original color and texture of the sill.

11) Apply the Finish Coat

Smooth on the finish coat with a trowel.

12) Wait for the Finish Coat to Dry

Allow the finish coat to dry to a leather finish. The mortar should be cool to the touch, firm, but not bone dry. You should be able to etch it with a fingernail.

13) Smooth the Surfaces

Smooth surfaces and any ridges left from the mechanical keys with a damp sponge or with a towel-wrapped dowel.

Things You Will Need


As a rule of thumb, it’s always best not to use harsh chemicals when cleaning sandstone – there’s always a chance they may react adversely to the stone and cause funny blotching to your valuable flooring. However, the use of diluted liquid chlorine is often used to help kill bacteria in the stone and remove stains.

If you wish to mop your tiles or pavers, we recommend using a string mop – they’ll easily get into the little crevices and nooks of textured sandstone surfaces to make sure they’re effectively cleaned. Also, make sure you don’t leave your sandstone to dry naturally: it’s a good idea to use a soft towel to dry sandstone after you clean any marks, stains or spills.

You may need to consider using a dedicated sandstone cleaner or liquid chlorine solution if your tiles or pavers become excessively soiled or stained. We recommend vacuuming the surface first, then spraying a light later of chlorine or cleaner onto the surface of the material, letting it soak for half an hour and rinsing the stone with a light hose (do not use a high pressure hose as this will damage the stone!) and broom or soft brush to gently push the sandstone cleaner into the pores of the stone. Once you’re done, make sure you rinse the surface and allow to dry.

This is general information and we do not take responsibility for any damages that arise from sealing or cleaning materials.

Cleaning Indian sandstone is simple. Use soapy water, or, for tougher stains as well as moss and algae, a bleach type mixture 
Breathable Stone Repair: The Perfect Solution
  • At this point, you may probably be wondering “if not cement, what else then? What are the suitable sandstone repair products one can use?” Lime based products are perhaps some of the most effective in repairing damaged stone. Before the advent of Portland cement in the 19th century, natural lime was traditionally used as a binder in mortars and plasters. And today, its popularity seems to have returned as more and more people become aware of its benefits. There are two different types of lime used in stone repair – hydraulic lime and non-hydraulic lime. Hydraulic lime usually appears in the form of bagged powder and is therefore much easier for builders as it is similar to working with cement. On the other hand, non-hydraulic lime is in putty form and thus, softer, slower to set and a bit trickier to use.
  • High-pressure washing is effective for sandstone cleaning. However, using extremely high pressure on a sandstone surface can destroy the surface, leaving it rough and ugly. The best way to go about it is by using soft pressure, which allows deep cleaning.
  • The stone paving slabs that are used on the steps are made from sandstone. This is quite a soft type of stone that allows it to be sanded down quite easily.
  • Rinse the sandstone and then sand the scratched area with 100-grit sandpaper. Rinse again and sand with 180-grit sandpaper. Rinse again and sand with 600-grit sandpaper to smooth out the stone. 


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