Alkaline burn is a phenomenon that occurs when fresh plaster, render, or cement-based substrates with high alkalinity (high pH) interact adversely with paint coatings. This reaction compromises the integrity of the paint film, leading to premature failure, discolouration, and a range of aesthetic and performance issues. In construction and refurbishment projects, understanding and mitigating alkaline burn is essential to achieving durable, attractive, and functional painted surfaces.
What Causes Alkaline Burn?
1. High Alkalinity of Fresh Plaster
Freshly plastered or rendered walls often have a pH above 11 due to the presence of free lime (calcium hydroxide). When water in the substrate evaporates, it brings soluble alkalis to the surface. These alkaline salts can react with certain paint components.
2. Paint Film Sensitivity
Many water-based and solvent-based coatings, particularly those not formulated to resist high pH environments, are susceptible to saponification (a chemical reaction between alkaline salts and fatty acids in the paint binder). This reaction can break down the paint film.
3. Excessive Moisture
High moisture levels in the plaster exacerbate the transport of alkaline salts to the paint interface, accelerating damage.
Symptoms of Alkaline Burn
• Discolouration
The most common sign is a yellowish, brownish, or whitish staining that appears in patches or as an overall tone across the surface. This is due to pigment and binder breakdown.
• Peeling and Flaking
Paint may lose adhesion as the alkaline environment degrades the binder, causing the film to detach.
• Powdery or Chalky Residue
The paint may appear to have a powdery surface or feel chalky to the touch, indicating binder degradation.
• Blistering
In severe cases, trapped moisture and alkaline salts can cause blistering and bubbling in the coating.
Key Factors Contributing to Alkaline Burn
• Insufficient Curing Time
Paint is applied before the plaster has adequately cured (generally 28 days minimum), leaving high alkalinity and moisture behind.
• No Alkali-Resistant Primer
Direct application of paint without an appropriate alkali-resistant primer.
• Inappropriate Paint System
Using a paint system not designed for high-alkaline substrates.
• Poor Surface Preparation
Failure to remove efflorescence and loose materials, leading to an unstable substrate.
How to Prevent Alkaline Burn
✅ Allow Proper Curing of Substrate
Ensure that the plaster or render has cured for at least 28 days. This allows most of the free lime to react and stabilise, reducing surface alkalinity.
✅ Check pH of Substrate
Use pH indicator papers or electronic meters to confirm that the substrate’s pH is ≤ 9 before painting.
✅ Apply an Alkali-Resistant Primer
Alkali-resistant primers (also called stabilising or sealer primers) create a barrier that resists alkaline attack. These primers often have modified acrylic or epoxy resins.
✅ Use pH-Tolerant Paints
Formulations based on acrylic resins, silicate, or silicone-modified resins have better alkali resistance than traditional emulsion paints.
✅ Good Moisture Management
Ensure substrates are dry, and address sources of rising damp or water ingress.
Repairing Alkaline Burn Damage
When alkaline burn has already occurred, follow these steps:
1. Assess Extent of Damage
Determine whether the damage is superficial (discolouration only) or if the paint film is compromised (peeling, flaking).
2. Remove Damaged Coating
Remove peeling or flaking paint by scraping, sanding, or water-jetting.
3. Neutralise the Alkalinity
Wash the surface with a mild acidic solution (such as diluted vinegar or a proprietary neutralising solution). Rinse thoroughly and allow to dry.
4. Reapply Alkali-Resistant Primer
Apply a suitable primer that can withstand high pH.
5. Repaint with Compatible Topcoat
Choose an exterior-grade, alkali-resistant paint.
Alkaline burn on painted plaster walls is a common challenge in construction, particularly in humid or damp environments. By understanding the chemistry of alkaline substrates and selecting appropriate coatings and primers, builders and applicators can ensure long-lasting and visually appealing paint finishes. Prevention is key—careful planning, proper curing, and material selection can significantly reduce the risk of alkaline burn and its associated problems.
