Short answer
Outdoors, sunlight, oxygen, heat and water act together on a paint film. Ultraviolet breaks down the binder at the surface, releasing pigment particles as a loose chalk; organic pigments fade; the surface roughens, so gloss falls and colours look paler and greyer. Photoactive titanium dioxide can speed the process, which is why exterior paints use coated rutile grades. Industry rates the damage on standard scales and tests it with natural and accelerated exposure, which do not always agree.
Polymer coatings degrade outdoors through the combined effect of sunlight, air, heat and moisture, which a review of plastics weathering describes as synergistic: each makes the others worse. Ultraviolet starts oxidation chains in the binder; heat speeds them; water swells and leaches the film, carries away degradation products and, in freeze–thaw cycles, stresses it mechanically. The results show up as colour fade, loss of gloss, surface cracking, chalking and, in some binders, yellowing. Coatings that face south (in the northern hemisphere) or are tilted towards the sky — roofs, sills, car bonnets — age fastest; vertical north faces slowest, which is why a single house can show several states of the same paint.
Chalking is the formation of loose powder on the surface as the binder around pigment particles erodes. White and pale paints make it obvious: a finger comes away white. On coloured paints, loose white particles scatter light over the colour, so the surface looks paler and hazier even if the pigment itself has not faded. Titanium dioxide's photoactivity matters here. Pure titanium dioxide, especially anatase, generates reactive species under UV that attack surrounding organic material, so pigment grades are coated with inert oxides such as silica and alumina; yet research still finds pigment photoreactivity one of the most important influences on coating degradation. Rutile grades with good surface treatment chalk slowly; poor grades, or interior paints used outside, chalk quickly.
Colour change outdoors has several components. Organic pigments of limited lightfastness fade, and bright reds, yellows and oranges tend to go first; inorganic oxides are usually stable. The binder may yellow, most visibly in whites. Gloss falls as the surface micro-roughens, and a matt surface reflects more white light diffusely, so the colour looks lighter and less saturated at every viewing angle. Dirt pickup adds grey. Distinguishing these matters for repair: cleaning can restore the part caused by dirt and some chalk, polishing can restore gloss on some car finishes, but faded pigment and eroded binder need repainting. The vehicles pillar covers how car finishes are layered to resist this, with a clear coat carrying UV absorbers above the colour.
Coating defects are rated on the ISO 4628 series, which grades blistering, rusting, cracking, flaking, chalking and corrosion around a scribe on 0–5 scales for quantity and size of defect; chalking is part 6. Weathering itself is tested by natural exposure at outdoor sites and by laboratory exposure to xenon-arc or fluorescent UV lamps with cycles of water, as in ISO 16474-3 for fluorescent UV lamps. Accelerated tests are fast but imperfect: a comparison of natural and laboratory weathering for waterborne acrylic coatings found that some standard tests produced far more degradation than natural exposure, while a tailored cyclic test gave changes resembling outdoor results. Accelerated results rank products; they do not predict years of life on a particular wall.
| Part | Defect rated | Scale |
|---|---|---|
| ISO 4628-2 | Blistering | Quantity and size, 0–5 |
| ISO 4628-3 | Rusting | Ri 0–5 |
| ISO 4628-4 | Cracking | Quantity and size, 0–5 |
| ISO 4628-5 | Flaking | Quantity and size, 0–5 |
| ISO 4628-6 | Chalking | Quantity and size, 0–5 |
| ISO 4628-8 | Corrosion around a scribe | Width in mm |
Why: Organic red pigment has faded and the binder surface has chalked, adding white haze.
Fix: Clean to see how much is chalk; repaint with a product rated for exterior use and strong sun.
Why: The chalk layer is a weak boundary under the new coat.
Fix: Remove chalk thoroughly and use a primer suitable for chalky surfaces before repainting.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Polymeric materials degrade outdoors through the synergistic effects of sunlight, air, heat and moisture, leading to loss of mechanical properties, fading, surface cracking, chalking and yellowing.
FactModerate evidence
To suppress titanium dioxide photoactivity while keeping UV shielding, pigment surfaces are coated with inert oxides such as SiO₂ and Al₂O₃, yet the photoreactivity of TiO₂ pigment remains one of the most important factors in coating degradation.
StandardModerate evidence
Coating degradation is rated on the ISO 4628 series, with chalking in part 6, using 0–5 scales for quantity and size of defects.
Caveat: Described via a research paper that applied the standard; the standard text itself was not consulted.
FactModerate evidence
For waterborne acrylic coatings, neutral salt spray and condensation-humidity tests caused degradation far exceeding that seen in natural exposure, while a tailored cyclic test produced changes similar to natural weathering.
Caveat: One family of coatings on steel.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.