I found this recipe, which sounds simple enough. I might use it for my new coop.
Calcium Hydroxide is an agricultural product used to modify soil properties and should be available quite cheaply (about $6 Canadian for 20kg) in a farm or agricultural supply depot.
Solution 1: Soak 3 kg Hydrated Lime (Calcium Hydroxide) in 8 litres water overnight to make a thick, creamy paste. This allows the calcium into the water and is referred to as 'slaked lime,' which is the basis for house plaster.
Solution 2: Totally dissolve 2 kg table salt in 10 litres water. The salt adds 'body' so that the whitewash will wear well.
When ready to apply, drain excess water from Solution 1 and thoroughly mix enough of Solution 2 into all of Solution 1 to achieve the desired consistency. Thicker is better than thin as you cannot thicken the end product by adding more calcium.
Thin with water if required, but do not thicken by adding more calcium as the added calcium must soak (slake) properly before using.
1 kg powdered animal glue may be added to improve adhesion and wearability. May be tinted with a water-based media if desired.