Ready Mix vs Traditional Plaster: Comparing Performance and Applications

Plaster is a versatile material used for construction, decoration and repair work. Traditionally, plasterers would mix the plaster ingredients on site from bulk raw materials. However, ready mix plaster offers a convenient pre-mixed alternative. This feature compares the performance and applications of traditional plaster mix vs modern ready mix plaster products.

What is traditional and ready mixed plaster?

Traditional plaster is a building material made from a mixture of a binder, such as lime or gypsum, and an aggregate like sand. These ingredients are mixed with water to create a workable paste which can be applied as ceiling or wall plaster. The exact composition and ratios can be adjusted based on factors like substrate and performance requirements. 

Ready-mix plaster is where all the dry ingredients such as gypsum, lime and additives like fibres or polymers are pre-blended. It typically comes in bags as a dry powdered mixture. Some products require water to the mix to create a workable plaster paste. Other products are available as ready mixed plaster skim in pails.

Ready mix vs traditional plaster: Pros and cons

When deciding between traditional plaster and ready mix plaster, you should weigh the pros and cons of each product. The following tables compare the 2 methods across several key factors. 

Traditional

Ready mix

How much do plasters contribute to sustainability?

Traditional and ready mix products have unique sustainability considerations related to material composition and production. Below, we compare the sustainability benefits and impacts of both plaster types.

Traditional plaster sustainability

• Natural ingredients - Traditional plaster is made from natural and renewable ingredients like lime, gypsum and sand that require relatively little processing.
• Low embodied energy - The simple mineral components and basic mixing process result in low embodied energy consumption.
• Recyclable/reusable - Plaster debris can potentially be recycled as a soil conditioner or reused as an ingredient in new mixes.
• Biodegradable - The all-natural mineral composition allows traditional plasters to fully biodegrade back into the environment over time.
• Low volatile organic compounds (VOCs) - Once cured, mineral plasters have little VOC off-gassing for better indoor air quality.
• Breathability - Plasters like lime allow the free movement of moisture vapour. This promotes air exchange and helps prevent issues like condensation and mould growth.

Ready mix plaster sustainability

• Embodied impacts - The industrial manufacturing processes and energy used to factory-blend ready-mixes often create higher embodied carbon and ecological impacts. 
• Chemical additives - Some polymers, admixtures and performance-enhancing additives used may have potential health or environmental impacts during their production or disposal. 
• Limited biodegradability - Many polymer-modified and cement-based ready mix plasters are limited in their ability to fully biodegrade. 

What are traditional and ready mix plasters used for?

Plaster is used across a range of construction and finishing applications. However, each plaster option is better suited for certain projects. Here, we outline the applications where both are best suited.

Traditional plaster applications

1. Interior walls and ceilings

Traditional gypsum and lime plastering create smooth, durable interior wall and ceiling surfaces. Their workability allows for achieving flawless, seamless finishes. Traditional plasters are ideal for historic renovations or new construction aiming to recreate a classic plaster appearance. 

2. Mouldings and architectural details

The workability of traditional plaster allows for intricate mouldings, ornaments and architectural details that require skilled hand-application. This makes it a superior choice for projects involving period architecture and historic plasterwork.

3. Rendering/stucco 

Lime-based renders and stucco plasters offer a breathable and strong adhesion. Their vapour permeability and flexibility make them well-suited as exterior weather-resistant cladding. Traditional renders are favoured for historic restorations requiring authentic stucco replacements.

4. Fireproofing

Gypsum-based traditional plaster forms dense coatings when applied at sufficient thickness. This gives them excellent fire resistance for protecting structural steel and other fire-rated assemblies from heat exposure.

Ready mix plaster applications

1. Drywall Interiors

Pre-mixed gypsum plasters are used to skim surfaces to create smooth plaster wall and ceiling finishes over drywall on interior projects. Their consistency helps in achieving flat, defect-free surfaces like lime plaster walls.

2. Renovations and repairs 

Ready mixed plaster formulations offer convenience for patching damaged plaster, skimming over existing substrates, or levelling surfaces during renovation projects. Their pre-blended consistency eliminates variation from mixing plaster on site. 

3. Tile substrates

Polymer-modified plasters create flat, rigid, dimensionally stable plaster surfaces that accept tile and stone finishes without cracks from shrinkage/movement. This makes them useful tile underlayment bases.

4. Exterior insulation finish systems (EIFS)

Polymer-modified plaster is used as the adhesive base coat to bond insulation boards to the substrate. It also serves as the embedded reinforced base coat and textured finish coat over the insulation. This provides a weatherproof exterior cladding surface.

5. High-performance fireproofing

Advanced formulas like vermiculite and perlite achieve fire resistance ratings of over 4 hours while providing impact and penetration resistance.

How to apply ready mix and traditional plaster

Application for traditional gypsum or lime plaster mix and ready mix plaster vary because of their material properties. The key distinctions in how each plaster type is applied are outlined below. 

Traditional plaster application

• Multiple coat application: Traditional plaster is applied in successive thin coats using hawk and trowel plastering tools. This multi-coat approach is necessary to build up the required thickness and allow each layer to set. 
• Scratch coat: The first "scratch" coat is scratched or scored to create a rough surface for the subsequent brown coat to key into mechanically.
• Brown coat: A thicker brown coat follows, levelling out any hollows or projections from the scratch coat.
• Finish coat: Once the brown coat is set, a thin finishing plaster coat is troweled on for a smooth and seamless surface.

Ready mix plaster application

• Single coat: Most ready-mix plaster formulations are designed for single-coat continuous application over substrates in one working session.
• Spray or pump application: Rather than troweling multiple coats, ready-mixes are often spray-applied using plaster spraying machines or pumped onto wall surfaces.
• Large continuous pours: Extended working times of ready-mixes allow large sections or full room volumes to be plastered without cold joints.
• Screeding and darbying: After spraying/pumping, plaster is screeded to the desired thickness and darbied (skimmed) for a smooth finish.

Is ready mix plaster waterproof?

An important consideration when selecting a plaster system is its resistance to moisture and water exposure. While traditional plasters like lime plaster can be fairly water-resistant, not all ready-mix plaster products are inherently waterproof.

Most standard ready mix plaster formulations are designed for interior applications and have limited waterproofing abilities. They can resist minor moisture exposure like high humidity, but prolonged contact with water may lead to degradation, delamination from substrates and potential mould or mildew growth. 

However, some manufacturers do offer specialised water-resistant ready-mix plaster varieties using advanced polymer additives and formulations. They are suitable for wet areas and exterior applications where the plaster may be exposed to rain or high moisture levels. 

Water-resistant ready-mixes contain polymers that reduce porosity and enhance adhesive capabilities to prevent delamination. Proper substrate preparation is still needed for maximum waterproofing.