Cow dung is good as an adhesive in small quantities -the clay plaster stick better to the surface. In larger quantities it hardens the plaster and when you add a lot of it, it even makes plaster water resistant (but not hydrophobic). If you want to make it hydrophobic, I would recommend using superhydrophobic nanopenetration for mineral faccades. It keeps the surface dry and permeable to thewater vapor, too.
