Apply plaster to a straw bale wall in three coats: press the scratch coat firmly into the straw and score it with a notched trowel so the next coat grips, let it cure, then build the brown coat up to a flat, even surface and finish with a thin coat that can be polished or textured. Keep the wall damp before each coat and keep lime coats damp after it, because fast drying leaves weak, cracked plaster.
Part of Straw Bale Construction
Application technique is the hand method of putting plaster coats on a bale wall so each bonds to the last.
Each Coat Has A Different Job
The first coat grips: it is pressed into the straw and scored to make a key. The second coat levels: it fills hollows and brings the surface to a flat plane. The third coat finishes: it is thin and takes the final texture. Rushing any one of them is how plaster fails. Earth and lime coats use the same sequence with different cure times; see Earth Plaster (Interior) and Lime Plaster (Exterior).
What You Need
- A prepared, dry, trimmed wall; see Surface Preparation and Mesh.
- Plaster of the right mix, in quantity for a whole coat of a wall in one session.
- Trowels, a hawk to hold plaster, a notched trowel for scoring, and a straight edge or screed.
- A sprayer.
- Eye protection and gloves.
Method: Three Coats In Order
Step 1: Wet the wall with a mist so it does not draw water from the plaster too fast.
Step 2: Scratch coat. Press plaster firmly into the straw with the trowel so it locks into the face. Earth plaster: 15-20 mm. Score the surface with a notched trowel for the next coat to grip.
Step 3: Cure. Earth plaster takes 1-2 weeks at minimum. Lime coats are kept damp for several days.
Step 4: Brown coat. Earth plaster: 10-15 mm. Build up to a flat, even surface using a straight edge.
Step 5: Finish coat. Earth plaster: 3-5 mm, polished smooth or textured.
Step 6: Close every gap so no straw is left exposed, especially around windows, doors and the junction with the roof.
Step 7: Protect the finished wall from rain, frost and strong sun until it is cured.
Common Mistakes
- Skipping the scoring. The next coat peels.
- Plastering the whole wall in a day in hot sun. The first areas dry before the last are finished.
- Thick coats in one go. They sag and crack.
- Not working the plaster into the straw.
Safety
Lime splash and overhead work
Lime plaster is strongly alkaline; wear goggles and gloves, especially when plastering above shoulder height where splashes fall toward your face, and flush skin or eyes with clean water for at least 30 minutes if splashed. Work from a stable scaffold, not a loose ladder.
What’s Next
- Learn next: Waterproofing and Fire Safety
- Combine with: Plastering and Finishing