Hardening and Waterproofing Existing Earth Plaster Walls

I have a 10 year old existing strawbale house with earth plaster in SW Texas. The plaster may not have been mixed or applied properly, as it erodes when rained on. It has a large overhanging porch, but when rain and wind come together the wind blows the rain onto the walls and a good quantity of the plaster washes off each time. My question: Is there any type of waterproofing paint or hardening mixture that can be applied to stop or slow this eroding problem? I would rather not apply a full re-plaster with lime or cement based plaster, but am open to all suggestions.

You could paint on a lime wash.
You probably want to keep the look of your earthen plaster, so I would;
mix up type s hydrated lime 1 50 lb bag into 2 5 gal buckets filled with 3 gal water
after mixing pour a little water on top and let sit for 2 weeks
mix earthen plaster in a large muck bucket then add equal amount of lime putty
thin with water until paint like
paint this mixture on the wall and trowel lightly while still wet
before this dries out, rub lightly with wet grouting sponge
you may have to do 2 coats to get a good hard outer layer, but not water proof you don't want that.
This will be much lighter than what you now have, but will resist water intrusion while allowing moisture to pass through and out of the wall.

Thanks Bill - Yes, waterproofing is the wrong word. Erosion-proofing would be more properly what I am looking for. Was wondering of a treatment of linseed oil might work and what the pros and cons of that might be.

Linseed oil would probably work, but I have no direct experience with that treatment, so I don't really know. I have seen others use it on interiors for the darkening effect that it gives, but for exteriors I use lime exclusively.
The earthen plasters that I have helped apply on historic adobe churches in New Mexico are re-applied every few years in a community event, so they are not stabilized at all. The oldest one, the Church of Santa Cruz, had a curve to the walls from all the slumped plaster with the bottom of the walls being maybe 10 feet thick after 300+ years of this.
I've attached a photo of our current restoration, an 87 year old house with the original lime render and plaster. We troweled on a new color coat and in the photo I am wetting the plaster with a grout sponge in order to fully carbonate the lime. This wall took maybe $20 in materials and will be good for another 50 years+.

Water glass (sodium or potassium silicate) will help bind the plaster. You can use silicate paint (i.e. Keim) if you want to change the color somewhat.

I know of several earth plastered houses where they treated the windward side with water glass or silicate paint - it does help.

I am on the brown coat phase of plastering for my straw bale and was gravitating towards using Kaolin clay as a final coat. Kaolin hardens like concrete but remains breathable and has a high degree of resistance to erosion. My research has led me to believe that lime plasters applied over earthen plasters have a high failure rate due to poor molecular bonding between the clay and lime. I am not a chemist, but the research seems to make sense and for that matter I have chosen the Kaolin clay route as bonding would not be an issue. Kaolin is also a local product in my parts (Georgia) so it is a choice I make with awareness in mind.

I have applied many different kaolin-based earthen plasters - mainly for the purpose of achieving lighter/brighter colors. I do not find that kaolin based earthen plasters are any more durable than plasters using other clays. So I'd have to disagree with Jeff on that suggestion.

I'd agree with him on the issue of lime adhering to earthen plaster though. It can be tricky to get lime to work with earthen plasters - in particular if you mix lime into earthen plasters. There is a threshold where too much lime effectively satisfies the ionic charges on the surfaces of the clay particles and breaks down the bonds between clay particles (van der Waals forces). When applying lime plaster on earthen plaster - the physical tooth is important. I don't have experience with lime wash on earthen plaster. I suspect it would work fine in most cases, but I would make test panels and abuse them to see how they behave.

Hi folks,

If I may share perhaps a more "traditional" and/or historical view of this aspect of the conversation. I do not believe, that the issue is whether cobbs, and other earthen matrix can work in concert with lime...it is a matter of "knowledge, applied understanding, and historical experience."

These two elements have been (still are in many areas of the world like the U.K. and Asia) used one upon the other and even mixed together...in good practice for a very long time. Here is but a VERY, very, very "short list" of descriptive modalities....

lime plaster mixed with cob

lime plaster over cob

I have done, and seen done...coutless modalities from "rice soup" additives to sealing with flax oil. The methods to try/test in a give area could fill several books.

Good luck (and reading/testing) to all,

j