• 2 1/4 pounds (1 kg) hydraulic lime
• 2 quarts (2 litres) water
• 3 1/2 ounces (100g) lime tolerant pigment

Note that only lime tolerant pigments can be used - this usually means earth pigments, ochres, umbers and the like. This recipe should cover about 130 - 260 square feet (12 to 24 square meters). Note that proportions are not exact. Some experimentation is required.

Slake the pigment. For a lighter color use less pigment but do not exceed the amount in the recipe.

Add water to the lime putty and sift to remove grit. Add pigment slowly with an electric mixer. Once mixed it can be stored indefinitly. Surface to be painted must be scrupulously clean.

At least 4 coats will be required, let each coat dry for 24 to 48 hours and spray with water to moisten before painting each successive coat.

It will appear dry and durty for a few days until it's taken in enough CO2 and has completed carbonation at which point it will be as of it were a coat of solid limestone.