This Help Center article is a summary of the official Earthaus PHL Binder Application Guide. Download the full PHL Binder Installation Guide (PDF).

Description

Earthaus BUILD PHL Binder is a pozzolanic hydraulic lime (PHL) plaster binder for scratch and brown base coats in both interior and exterior applications. It is compatible with natural building systems such as adobe, hempcrete, strawbale, and other earthen walls, as well as conventional substrates.

Typical system:

Scratch Coat: Creates a mechanical bond and texture (tooth) for the next coat.
Brown Coat: Builds thickness uniformly and establishes the final shape of the wall.

Coats are typically ⅜" thick, but thickness is substrate-specific. Always refer to the Wall System Guide for your specific substrate.

Safety Precautions

PHL Binder is strongly alkaline in both dry and wet form and can cause severe injury.

PPE required: gloves, eye protection, long sleeves, dust mask or respirator. Do not wear contact lenses when working with lime.
Eyes: flush immediately with copious cold water; seek immediate medical attention.
Skin: wash with plenty of cold water, then mild soap and water; vinegar may help neutralize lime if irritated.
Inhalation: move to fresh air; remain away from dust until symptoms subside.
Ingestion: do not ingest; seek medical attention immediately.

Project Planning & Preparation

Substrate Preparation:

The substrate must be structurally sound, stable, and free of dust, oil, loose material, or other contaminants that could inhibit bonding.
Some substrates (e.g., hempcrete) must be fully cured and dry before plastering. Cure time varies with thickness, temperature, and humidity — refer to the relevant Wall System Guide.
Dampen the substrate before plastering if indicated in your Wall System Guide, to prevent flash drying.

Application Conditions:

Do not apply if temperatures could reach freezing within 72 hours of application.
Apply in conditions that allow steady, even drying.
In active drying conditions (high heat, direct sun, wind), shade the plaster and mist to slow drying. Protect fresh plaster from direct rainfall.

Sourcing Sand Aggregate

PHL Binder requires locally sourced sand aggregate, available at sand and gravel yards, landscape suppliers, and masonry supply yards. Ask for sand produced to a recognized stucco/masonry/concrete standard (ASTM C897, C144, or C33).

Key properties:

Sharp, angular particles (not rounded), well-graded with a variety of sizes.
Upper particle-size limit ¼" (ideal ⅛").
Clean and free of clay, silt, dust, salts, and organic matter.

Mixing

Ratios (by volume):

Scratch Coat — 1 part binder : 1.5 parts sand
Brown Coat — 1 part binder : 2.5 parts sand

Recipe per 50 lb bag (1 bag = two 5-gallon buckets by volume; 1 bucket of sand ≈ 65 lbs):

Scratch Coat — 1 bag binder + 3 buckets sand + ~4 gallons water (starting amount)
Brown Coat — 1 bag binder + 5 buckets sand + ~5 gallons water (starting amount)

Water is a starting amount only — sand moisture varies, so adjust to consistency.

Mixing steps:

With the mixer running, add the starting water.
Add half the sand.
Add the bag of binder.
Check consistency; add water if needed.
Add the remaining sand; check again and adjust water.
Once the consistency is right, mix for 10 minutes.

Target consistency: holds in a soft pile like soft-serve ice cream — it stays on a hawk without sliding off when tipped to about 30°. Too wet = runny like a milkshake. Too dry = stiff and crumbly. Avoid overwatering, which weakens the plaster.

Scratch Coat Application

Dampen the substrate first if indicated in your Wall System Guide.
Apply an even coat to the thickness specified in your Wall System Guide. Press firmly into the substrate; do not overwork the wet plaster, which can separate the sand from the binder.
While the plaster is still wet, scratch it horizontally ¼" deep with a scarifier to create tooth for the brown coat.
Mist or shade to prevent rapid drying.
Allow 10 days for the scratch coat to cure before applying the brown coat.

Brown Coat Application

Dampen the cured scratch coat before applying — a dry scratch coat is highly absorbent. Walls should be darkened and damp but with no standing water.
Apply an even coat to the thickness specified in your Wall System Guide. Maximum ½" per coat. To build out uneven walls, apply successive ½" layers, scratching and curing 10 days between each layer.
Recommended — Fiberglass Reinforcing Mesh (reduces cracking): apply half the coat thickness, embed the mesh flat (overlap joints ≥ 2"), then apply the remaining half to fully cover it.
Sponge-float the surface once the plaster firms up (difficult to dent with a thumb, easy to scratch with a fingernail) to flatten and even it.
For Lime Stucco Fine, Sand, or Stone finishes, press the sand grains smooth with a trowel at the right moment. For Lime Stucco Coarse, sponge-floating is the final step.
Allow 10 days for the brown coat to cure before applying a finish.

Curing

Keep scratch and brown coats moist for at least 48 hours after application. Mist morning and evening when walls are cool, and wait until the plaster is "thumbprint hard" so misting doesn't erode the surface.
In hot, sunny, or windy conditions, shade with tarps and mist more often. In cool, humid conditions, misting may not be needed in the first 48 hours. Protect from direct rainfall.
Allow 10 days of curing between coats — scratch → brown, and brown → finish. Applying too early weakens the plaster system.
Plaster continues to gain strength over time as it absorbs CO₂ from the air (carbonation), reverting toward limestone.

Finishes & Additional Coats

Compatible Earthaus finishes include Lime Stucco, Sand, and Stone.
Once the brown coat has cured 10 days, the base coats are ready for a finish. See the relevant finish installation guide.

Troubleshooting

Cracking: caused by coats applied too thick (over ½"), poor or too-fine sand, or rapid drying. Hairline cracks under 1/16" are typically superficial; cracks over 1/16" may indicate structural movement and should be evaluated by a professional.
Dusting / weak surface: overwatering or inadequate mixing.
Delamination: substrate too dry, dirty, or poorly prepared.

Storage & Shelf Life

Unmixed bags: store dry and in low humidity, up to 1 year.
Mixed plaster: store covered at room temperature for up to 5 days. To return to working consistency, remix for about 5 minutes (add a little water if needed).

Calculate How Much You Need

Estimate how much PHL Binder your project needs with the PHL Binder Calculator — select your wall system in the calculator.

Customer Support

For product questions, email [email protected]. For project-specific guidance, schedule an Earthaus technical consultation.