Why Exterior Paint Fails: UV Degradation vs Alkali Attack

Exterior paint failure is one of those home-maintenance headaches that looks simple on the surface- “my paint is peeling, let’s repaint it.”
But the truth? Two completely different forces may be destroying your walls:

UV degradation from sunlight
Alkali attack from fresh or poorly cured cement

Both create peeling, fading, chalking, and discolouration…
But each needs a different fix. Repainting blindly can make the problem worse.

Below is a clear, homeowner-friendly guide (with technical depth) on how to correctly diagnose what’s happening before you spend money on repainting.

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UV Degradation – What It Looks Like & Why It Happens

You’ve seen this:
Paint fades unevenly, looks dull, becomes powdery, or completely loses its original colour.

That’s UV degradation, and it happens because:

Sunlight breaks down the binder that holds pigment together
Pigments lose their colour stability
Resin weakens and turns chalky
Dark colours fade faster because they absorb more heat

How to identify UV degradation:

Rub the wall; if colour comes off like powder → chalking
Extremely faded patches on sun-exposed sides
No dampness or bubbling- just dryness and dullness
Paint surface feels dusty or brittle

Most common areas:
South-west facing walls → maximum UV exposure
Parapets, exposed balconies, roof walls

UV damage = age + sunlight, not cement reactions.

If your exterior walls already show dullness or chalking, a professional exterior painting service can restore the finish with UV-stable coatings while preventing future sun damage.

Alkali Attack – A Deeper, Chemical Problem Most Homeowners Miss

This is the one painters often blame on “bad paint,” but the real cause is your wall chemistry.

Fresh cement, new plaster, or damp walls release free lime (alkali).
When paint touches this alkalinity, the binders break down → causing burning, patchiness, and peeling.

What does alkali attack look like?

Yellowish or brownish stains pushing through the paint
Blistering, flaking, or peeling in patches
Paint looks “burnt” or uneven within months of application
Efflorescence (white salt crystals) visible near peeled areas

Quick wall test:
Put a pH strip on a dampened patch.

pH 8 or below → safe
pH 9-11 → risky for painting
pH 12+ → guaranteed alkali attack

Most common scenarios:

Painting over new plaster within 30-45 days
Using poor-quality primer or skipping primer
Moisture seepage causing the lime to migrate outward
Repainting without removing chalking or salts

If your wall shows repeated moisture-driven alkali damage, targeted waterproofing services help stabilise the surface before repainting.

Alkali attack = chemical destruction, not sunlight.

UV vs Alkali – The Side-by-Side Diagnosis Guide

Below is a quick cheat sheet that homeowners love referencing:

Symptom	UV Degradation	Alkali Attack
Chalking	Yes	Rare
Fading	Strong indicator	Not related
Patchy peeling	No	Common
Stains (yellow/brown)	No	Yes
Powdery feel	Yes	Sometimes with efflorescence
Happening on new plaster	No	Very likely
Happens only on sun-exposed areas	Yes	Not related
Efflorescence	No	Yes

Even painters confuse these two, which is why misdiagnosis is so common. A professional home inspection can identify whether the root cause is UV exposure, moisture, or alkaline salts- before you invest in repainting.

How to Fix Each Problem Correctly Before Repainting

If it’s UV degradation → solution is straightforward

Wash or scrub chalking off
Apply UV-resistant exterior primer
Choose exterior paints with higher UV stability (acrylic, silicone-based)
Prefer lighter colours to reduce fade
Repaint every 4–6 years in harsh climates

Best suited products:
100% acrylic exterior paints, elastomeric coats, or silicone-enhanced finishes.

If it’s alkali attack → you need corrective steps

This one needs more precision:

Remove all loose paint completely
Wash the wall to remove salts
Allow the wall to dry fully
Test pH again (must be under 9)
Apply alkali-resistant primer (mandatory)
Finish with exterior paint

If pH doesn’t drop even after drying →
You’re likely dealing with rising damp, and waterproofing repairs are needed before repainting.

When to call a professional?

If the paint keeps peeling despite repainting
If efflorescence returns repeatedly
If wall moisture levels are unclear
If the home is near coastal or high-humidity areas

Correct diagnosis = cost savings + longer paint life. If structural cracks are allowing moisture to enter the walls, wall crack filling services ensure a stable surface before applying fresh exterior paint.

The Final Word

At the end of the day, exterior paint failure isn’t just cosmetic- it’s a message from your wall.
UV degradation and alkali attack look similar, but solving them requires completely different approaches. If you misdiagnose, you waste time, money, and labour… and the problem comes back within months.

At Clean Fanatics, we inspect the wall scientifically- checking pH, moisture, UV exposure patterns, and existing paint chemistry- so you know exactly what your wall needs before you repaint. Smart diagnosis leads to durable results