DIY Plastering 
Interior and Exterior

DIY plastering with natural plaster on a straw bale wall can be simple if you have prepped your walls well. We used the three layer method – scratch coat, brown coat, and finish coat.

Before plastering a Nebraska style straw bale home, it is important to let the bales settle. You will avoid patching cracks in prematurely plastered walls. We let ours settle about three weeks and have had very little cracking to repair.

The key to plastering your straw bale home is in the preparation. Trimming your walls with a string trimmer, filling the spaces between bales thoroughly, bridging the joint between wood and straw bale with plastered burlap, and spraying aliz to give the straw bale wall additional “tooth” all contributed to a straightforward plaster job.

DIY Plastering Tools

Tools for exterior and interior diy plastering include trowels, wall paper paste brush, hawk, wheelbarrow and mixer.

Choose a trowel for application that matches your strength and the thickness of the mud. For rough applications such as the scratch coat, I used a long (14”) rectangular finishing trowel or the longer of my Japanese trowels.

Close to windows and extending rafters, I used a smaller, more maneuverable trowel. The final troweling was done with a flexible pool trowel or, on the exterior finish coat, a brush.

Scratch Coat

Our scratch coat was made keeping in mind adhesion to the prepped wall and the ability to bond well with the brown coat. In all plaster applications, we used a hose with a mist setting on the nozzle to wet the wall before applying new plaster.

We mixed a rough adobe, screened through 1/4" hardware cloth, with a heavy percentage of chopped straw. Run your straw through a string mulcher twice and, if you  are using a string trimmer in a tub, try to get your straw pieces fine and short.

A mixture of 10 to 12 shovels of clay strained through 1/4" mesh with two gallons (approx.) of fine chopped straw is a good place to start.

Using a three bale practice wall or just being willing to adjust as you go will help you refine your process. Remember, it doesn’t matter much if this layer cracks a bit and it is good if there is straw sticking out. This just helps the next layer hold.

Cracking can be reduced by controlling the amount of liquid in the mix. The more water in your plaster, the greater the likelihood of cracking.

Some plasterers like to score this coat, hence the name “scratch coat”, with either a trowel or a garden rake making furrows parallel to the ground in the wet plaster. This gives even more bonding surface to the brown coat.

Brown Coat

Screening our adobe clay through a 1/8” hardware cloth frame into a tub gave us a richer plaster mix; that is, we had a higher percentage of clay in the plaster. Again we used very fine straw and less water to reduce cracking.

This is the last real opportunity for a smooth wall, so if there are still low spots on the wall, don’t try to make this layer too thick as cracking will result in most cases. Let the plaster set up overnight and apply a second layer to the wall.

The result should be a wall that is without major low or high spots with minimal cracking. We allow the final application of “brown coat” to dry for a couple days protected from direct sun. For our job, that was easy with the porch roof providing shade.

You might hang a tarp over the wall and even use the ‘mist’ setting on your garden hose to slow the drying process and help minimize cracking.

Finish Coat – Texture and Color

The exterior finish coat on our final building, our house, was the most successful. The color of our clay is a reddish brown and we loved it, so we chose not to add color to our final exterior coat.

There are a number of sources for natural pigments that can be added to clay plaster. See our book list.

We found them a bit expensive for large areas of diy plastering, although we did add iron oxide to the interior plaster of our utility building. This produced a very light reddish color that fit well in a Southwest interior.

It is very important to measure your color pigments carefully and to mix very thoroughly or you might have unwanted variations in the color of the wall or streaks of pure unblended pigment.

Exterior finish coat was simply aliz mixed with water to about the consistency of thick paint. Again, we used a 1/2” drill with a paint paddle to mix the aliz in a 5 gallon bucket.

After some experimentation with the sprayer and a finishing trowel, we elected to paint on the aliz with a wallpaper pasting brush. The brush made working plaster into small cracks easy and we liked the resulting rough texture on the finished wall.

We troweled the interior finish coat over the brown coat in a very thin layer and then went over the wet wall with a pool trowel.

 After the wall was cured, we used a paint roller and applied a layer of thin flour paste as a surface hardener.

Interior finish coat was a blend of Kaolin clay, our local adobe clay, and 60-grit sand. The Kaolin clay was in bags and had been mined in northern AZ and purchased from a local pottery supply store. Our adobe was strained to aliz and the 60-grit “blasting” sand came from our local hardware store.

For hardness, a flour paste can be added to the mix although the color of our wall where this was done our walls were darker. I suggest using a mix on your practice wall and writing down the combination of materials that you find producing your best mix.

We applied a finish coat over a sprayed aliz coat on the sheet rock of our interior walls to match the straw bale walls. It worked out well.

While these materials and methods worked very well for us, you might explore alternatives suggested in the books available through our book store. We wish you well in your diy plastering venture.

Happy Building!

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