Saponification
In alkyd or solvent-based systems, oils react with alkalis and break down into a soapy mass. The film loses its structural integrity from within.
Plaster Alkalinity & Impact on Paint
Why fresh plaster can destroy a paint system
What is alkalinity
Fresh plaster and concrete contain calcium hydroxide. During curing, the pH can reach 12–13 - an extremely alkaline environment.
At such high pH, certain resins are chemically affected. Painting over uncured plaster can weaken the film before it even has a chance to perform.
What high alkalinity causes
Alkalis don't just ruin appearance - they cause chemical breakdown of the paint system. Four distinct damage mechanisms stand out.
Reduced adhesion
The film doesn't bond properly with the alkaline substrate. Delamination appears within weeks instead of years.
Discolouration & dullness
The surface shows uneven tone, localised dullness, or colour shift - especially where curing happened at different rates.
Blistering & delamination
Especially combined with moisture, alkaline salts migrate to the surface and build pressure beneath the film.
When the risk is highest
Alkalinity isn't always a problem - it becomes one when painting happens too early, before the substrate has cured sufficiently.
Fresh plaster
Not sufficiently cured. The rush for "handover" means painting over substrates at pH 12+.
Local repairs
Plaster patches on an old facade. The new plaster is alkaline while the rest of the wall has cured - creating an inconsistent base.
Fresh concrete
Recently poured beams, columns, or walls. High alkalinity combined with low porosity makes adhesion even harder.
Rushed handover
Painting under time pressure. Curing time is "sacrificed" - and failure appears within months.
How to handle it properly
| Measure | What it does | When to apply |
|---|---|---|
| Curing time | Lets the pH drop to safe levels (~9) | 28+ days after plastering |
| Alkali-resistant primer | Creates a barrier between alkaline substrate and paint | Always on fresh plaster/concrete |
| Moisture check | Prevents alkaline salt migration to the surface | Before every application |
Premature painting on fresh plaster is one of the most common causes of early failure - and the easiest to prevent.
Why modern acrylics resist better
100% acrylic systems show significantly greater resistance to alkaline environments. Their chemical stability means they don't saponify and maintain adhesion even at high pH.
This is a key reason why alkyds are no longer used on exterior facades - on fresh plaster, saponification is virtually guaranteed.
How to recognise an alkalinity problem
Alkalinity is invisible - but its symptoms appear early. If any of the following are spotted, a technical assessment is needed.
Uneven paint cure
Some areas look "cured" while others remain dull or tacky - a sign of chemical reaction with the substrate.
Local delamination
Paint peels at repaired spots or on fresh plaster, while it remains stable on the older wall.
Efflorescence
White crystalline deposits on the surface - alkaline salts transported by moisture and deposited on top of the film.
Premature wear without moisture
Paint deteriorates within months with no visible moisture problem - the cause is chemical, not mechanical.
Related Articles
Primers: When Are They Really Necessary? How Does Moisture Affect Paint? Capillary Rise & Moisture Transport Painting New Plaster or Plasterboard
Paints for Building Facades
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