PRC (Paper Reinforced Clay) Plaster - How to apply plaster over drywall, cob, earthship, or anything

This clay plaster is user-friendly. It is easy to apply and to finish. It can also be used over many wall types. This mix is high in fiber from the toilet paper, making it crack resistant and flexible unlike other masonry mixes.

It is clay rich, so has very high-water retention and water resistance. This also makes it slow drying and easy to work with.

Suitable areas:
Interior walls
Exterior walls with low weather exposure
NOT suitable for showers, floors, or surfaces exposed to high abrasion or water

Recipe
Sand (aggregate)
I used calcium carbonate sand, (aka lime sand or ag-lime), from a pre-blended 50-200 grit bag. It is difficult to find, but I like how lightweight it keeps the mix. Other sands can be substituted.
The sand size will affect the final thickness and color of each coat. This plaster should be applied no more than 1½ times the thickness of your sand grain-size.
Clay (bonder)
I used Lincoln fire clay, a common pottery clay. There are plenty of bagged clays, or you can screen and process your own soil.
For softer clays, such as Kaolin, I would recommend adding 10-20% bentonite clay for strength.
TP (fiber)
I use toilet paper, as a paper fiber for fine plasters. Any paper can be used, including shredded cardboard or newspaper, but the toilet paper is easier to break down with the drill and free of dye and labels.
Admixes
Glue – Any glue can be added for water resistance and extra hardness. Most glues, including wheatpaste, can make it harder to work and can cause cracking and discoloration.
Darvan – Two kinds that I use are Darvan 7 and Darvan 811. This is a deflocculant that changes the molecular charge of the clay, making the clay platelets repel one another. This gives it more water retention and resistance. Darvan makes the plaster easier to work and apply evenly. It also reduces shrinkage and cracking.

Mix Examples
These are rough examples and may vary depending on what material you are using. They should be tested and adjusted. Don't forget to experiment!

Mix used in video:
1 cup clay (200g)
1 cup lime sand 50-200 grit (350g)
10 sheets of TP (5-7g)
Darvan (1-2g)
Water 3/4-1 cup (170-210g) or as needed

Alternative sand options:
Every sand gives a different color and characteristic, greatly affecting the final look.
Barnlime
1 cup Barnlime (400g), screened through window-screen
½ silica sand (165g)
https://www.tractorsupply.com/tsc/product/waukesha-lime-barnlime-50-lb-bag
Pulverized Dolomitic Limestone
1 cup Dolomitic Limestone (350g) https://tinyurl.com/y85w5ygl
More TP needed: 20 sheets (10-13g)
https://tinyurl.com/y85w5ygl
Masonry (regular) sand
1 cup sand (330g), window-screened

Wall Prep
Over clay wall or concrete masonry – No need for any primer or glue. Hydrating the substrate will allow more working time.
Over drywall or wood/plywood walls – These should be primed before applying. Use any conventional primer and prepare as you would for painting. Then use wheat paste to help bond (1:1 wheat paste concentrate to water - see ** description). Or use sand in your primer. These should be dry before applying the plaster.

Applying
The mix should be wet enough so that it is easy and effortless to apply thin.
You can apply as many coats as needed, typically 2 is the minimum.
Allowing the wall to dry in between coats helps reduce shrinkage and increase consistency.

Sponging or Compressing
Both these steps help bring consistency to the texture of the wall, by either compressing or moving loose aggregate from the surface into the pores.
Sponging – This is an especially user-friendly way to get rid of large trowel marks or wrongdoings. Should be done with clean water and a lightly damp sponge. Make circular patterns to smooth and blend in irregularities. If the sponge ,is too wet you may leave strokes (which can be wiped out later) or cause excess surface erosion.
Compressing – with a trowel and extra moisture (spray bottle or brush with water), this pushes any leftover aggregates into the wall and brings the cream/clay to the surface. Beware this may burnish the surface and leave darker patches in light plaster if not desired. This can be avoided by using a plastic trowel. Compression is time-sensitive and is best done at the “leather-hard” stage.

Fixing
Save some of your original plaster for patches. You can dry it and store for however long you like. Reconstitute with water when you need to patch, wet the area of the patch, allowing it to fully hydrate before applying. Be aware it is much harder to patch over sealed walls.

Sealing and tools - continued in google doc https://tinyurl.com/y8ocze84

Видео PRC (Paper Reinforced Clay) Plaster - How to apply plaster over drywall, cob, earthship, or anything канала The Nito Project