Chalk paint is latex paint with calcium carbonate added in the form of baking soda, grout, or plaster of Paris to act as a porous bonding agent. When it dries it resembles a chalkboard that can be written on with chalk but this only works when there is no additional finish on the surface.
The biggest attraction is that it doesn’t need primer, and will often require fewer coats to cover. It will adhere better to non-porous surfaces than milk paint does.
The biggest downside is that it requires waxing to protect the finish, and waxing can be a very time consuming task that takes practice to achieve good results. In addition, regular maintenance waxing may be required.
Milk paint is a water-based mix of milk, lime and colour pigments used primarily to give an antique look to furniture. With milk as the main ingredient, it must be used within a few days, even if refrigerated.
The milk binds the colour pigments in the same way that polymers do in latex paints and oils do in oil-based ones. It doesn’t give off noxious vapors (often called VOCs – volatile organic compounds) so is appealing to today’s green community.
Milk paint is designed to seep into porous materials so that it won’t chip or peel. If it is to be painted on non-porous materials, it will need bonding additives to adhere.
There is a lot of information in this fascinating page called A Brief History of Milk Paint which reviews its beginnings thousands of years ago through its transitions to the recipes used today.
Water is added to milk paint to achieve the desired consistency: less creates a translucent colour like stains while more produces a thicker layer. Any mixture will likely need regular stirring to prevent settling during use.
Most milk paint comes in a powder form that must be mixed in a blender along with bonding agents if required.
If you are interested in trying it, this link has a recipe Milk Paint Recipe.