You can apply Earthaus plasters to cement backer board (such as HardieBacker or Durock) and magnesium oxide (MgO) board when the surface is properly finished and primed. These substrates are suitable for stable, dry interior environments — not for direct wet-area use except with the Earthaus Tadelakt System.
Surface Preparation Requirements
Cement and magnesium boards are rigid, non-combustible substrates commonly used in high-humidity spaces. Before plastering, they must be finished to a Level 4 drywall standard, ensuring a smooth, even surface.
Preparation steps:
Finish all joints and fasteners using standard drywall finishing techniques.
Sand smooth once dry, ensuring no ridges or raised edges.
Prime the entire surface with a water-based PVA or latex primer mixed with Earthaus Primer Grit to create a mechanical bonding texture.
Why it matters: These boards are dense and have limited suction. Primer Grit improves adhesion and provides the necessary surface texture for lime plaster bonding.
Wet Area Limitations
While cement and MgO boards are often used in high-humidity areas, not all Earthaus plasters are rated for wet-area installations:
Only the Earthaus Tadelakt System is approved for use in wet areas such as showers or tub surrounds.
Do not apply plasters over cement or MgO boards that are exposed to direct water spray or prolonged moisture.
Lime plasters are breathable and moisture-tolerant, but they rely on a stable, dry substrate for durability. In wet environments, proper vapor management and system design are critical.
Before You Start
Ensure the substrate is clean, dry, and free of dust before priming.
Primer Grit is essential for achieving the proper mechanical key on smooth or low-absorption boards.
Follow all applicable local building codes for substrate and vapor barrier requirements in high-humidity areas.
Questions?
Email support@earthausplaster.com or text 1-800-917-2414 — our team is happy to help.