posted 6 years ago
Hi Sunny,
I'm not familiar with wheat paste's water resistance characteristics--I have only ever added it (or any flour paste or gluten) to an interior finish clay plaster or alis to prevent dusting. In general, flour pastes or gluten are a relatively small portion of the mix. I'll guess that if it does inhibit water vapor movement it's not significant, and that if it offers some resistance to erosion from wind-driven rain, it's also not significant. Water soluble finishes like clay (earth) plaster or alis on an exposed exterior wall will need relatively frequent (by modern standards) maintenance. I'm also not familiar with how well an alis would stick to a lime wash. You might do some testing first--apply it to a section of one of your walls and watch it for a season, maybe spray it with a garden hose to simulate wind-driven rain. Another New Society Publishers book, Essential Natural Plasters, gets much further into the weeds on both interior and exterior wall finishes than the CASBA book's chapter on plastering straw bale walls--you might find more information there?