Quick answer
Composite door discolouration usually falls into one of four types: green algae staining on damp north-facing doors, black mould spots, a white salt haze from sea air, or overall UV dulling. Each needs a different treatment, so diagnosing the type correctly before you clean saves wasted effort and avoids damaging the finish.
Overview
Discolouration comes in green (algae), black (mould), white (salt) or dull (UV). Each has a different treatment.
Symptoms
- Green stains - north-facing damp.
- Black spots - mould.
- White haze - salt.
- Overall dulling - UV.
Common causes
- Coastal exposure.
- North-facing damp.
- UV.
The four types of discolouration and how to tell them apart
Green discolouration is almost always algae, and it appears most on north-facing doors that stay damp longer after rain because they get little direct sun to dry them out. It often follows the pattern of water run-off, streakier at the top and heavier lower down or around the frame edges.
Black spotting is typically mould rather than algae, and tends to cluster in sheltered corners, around seals and in recessed panel detailing where air doesn't circulate well. A white, slightly gritty haze is salt deposit from sea spray and evaporating moisture, most common on doors within a mile or two of the coast, and it wipes to a paste rather than staining the surface itself. General dulling without any obvious pattern, affecting the whole door evenly, points to UV fade rather than a biological or salt cause.
- Green streaks, worse on north-facing doors: algae.
- Black spots in corners and recesses: mould.
- White gritty haze near the coast: salt deposit.
- Even dulling across the whole door: UV fade.
Treating algae and mould without damaging the finish
Start with warm water and a mild detergent applied with a soft cloth or sponge, working in sections rather than trying to tackle the whole door at once. This shifts a surprising amount of algae and light mould on its own, especially if you catch it before it's had months to bed in.
For anything that doesn't lift with soapy water, a dedicated non-abrasive composite door cleaner formulated for GRP surfaces is the next step up. These are designed to break down biological growth without the harsh chemical action that damages a gel-coat finish or degrades the seals around glazing and hardware.
Never use bleach on a composite door. It's tempting because it's cheap and effective on algae elsewhere in the garden, but bleach can damage the GRP finish over repeated use and will degrade rubber seals faster than normal weathering would.
Dealing with salt haze near the coast
Salt haze is the easiest of the four to treat because it's a surface deposit rather than a stain that's bonded to the material. A warm water rinse followed by a proper wash with mild detergent removes it completely in most cases, without needing anything stronger.
The issue with salt is frequency rather than difficulty. Properties within a mile or so of open coast can see salt build-up return within just a few weeks in exposed, windy conditions, so the practical answer is more regular light cleaning rather than a stronger cleaning product.
Why north-facing doors discolour faster in Cornwall
A north-facing door in Cornwall's damp, mild climate rarely gets a proper drying-out period, especially through autumn and winter when low sun angles mean direct sunlight barely reaches some elevations at all. That persistent dampness is exactly what algae and mould need to establish themselves, which is why we see disproportionately more discolouration call-outs for north-facing doors than any other orientation.
If your door sits under an overhanging porch roof that further reduces what little sun it does get, that combination of shade and shelter from wind (which would otherwise help it dry) makes it close to the worst-case scenario for biological staining.
A realistic cleaning schedule to prevent recurrence
For most Cornwall properties, washing every three months keeps discolouration from building to a visible level. Properties directly on the coast, or with a north-facing door in a sheltered position, benefit from monthly attention instead, since the conditions that cause staining redevelop faster there.
An annual wax after cleaning adds a layer that makes the surface less hospitable to algae taking hold in the first place, as well as helping shed the salt film faster on future washes. It's a modest extra step that measurably reduces how often you need to do a harder clean.
When discolouration might signal a coating failure
Occasionally what looks like stubborn discolouration doesn't respond at all to cleaning, even a proper wash and a dedicated composite cleaner, and instead looks like it's actually within the finish rather than on top of it. That's worth having assessed rather than persisting with, since it can indicate an early coating issue rather than a cleaning problem, and repeated scrubbing on a compromised finish can make things worse.
We can tell you on inspection whether what you're dealing with is a stubborn but ordinary discolouration case or something that needs a different conversation about the door's finish.
Pressure washing: what's actually safe
A pressure washer can help shift ingrained algae on a badly affected door, but only used carefully. Keep it on a low-pressure fan-tip setting, held at least 30cm from the surface, and never aim directly at seals, the letterbox, or the gaps around glazing beads and hardware, where high-pressure water can force its way past a seal that would otherwise cope fine with normal rain.
If in doubt, a soft-bristle brush and a bucket does the job just as well without any risk to the seals, and takes barely longer for a single front door.
What the guarantee covers versus everyday discolouration
The colour-fastness guarantee is built around gradual, even UV-driven fade measured against a sheltered reference point, not the patchy, uneven discolouration that comes from algae, sap, exhaust residue or a poorly rinsed cleaning product. Those causes sit outside guarantee territory because they are environmental staining rather than a change in the pigment itself, and in most cases they clean off rather than needing a claim at all.
Where it gets more useful is when discolouration looks uneven precisely because one part of the door has been shielded from UV by a porch canopy or a hanging basket, while the rest has weathered normally. That kind of patchy fade is still assessed under the guarantee, since it is UV-driven, just unevenly distributed.
Seasonal causes of discolouration through the year
Spring brings pollen and tree sap discolouration, particularly under sycamores and pines common in Cornish gardens, leaving a sticky yellowish film that stains if left to bake on in the first warm spell. Summer's main culprit is a build-up of sun cream and insect repellent residue transferred from hands onto handles and the surrounding panel during the busier visitor season.
Autumn brings damp leaf litter sitting against the bottom of the door if it is not cleared promptly, which can leave a tannin stain on lighter colours within days. Winter's biggest cause is road grit and salt spray thrown up from nearby lanes during storms, settling into a grey film across the lower half of the door.
- Spring: tree sap and pollen film, wash off before it bakes on.
- Summer: sun cream and repellent residue around the handle area.
- Autumn: tannin staining from damp leaves left against the door.
- Winter: road salt and grit thrown up by passing traffic.
North-facing doors and organic discolouration
North-facing doors that stay shaded and damp for longer are far more prone to a greenish or black organic discolouration from algae and lichen than sun-exposed south-facing doors, which dry out quickly enough to discourage growth. This is a different mechanism entirely to UV fade and needs a different response, focused on removing moisture and organic film rather than protecting pigment from sunlight.
If your door sits under a tree, beside damp render, or in a permanently shaded porch, building a slightly more frequent clean into your routine, perhaps monthly rather than quarterly, keeps organic discolouration from establishing a real foothold in the gel-coat's texture.
When discolouration is beyond a clean and needs assessment
Most discolouration lifts with warm soapy water and a soft brush, but if a stain has been left for an extended period, particularly tannin from leaves or algae that has been growing for more than a season, it can etch slightly into the surface texture of the gel-coat and leave a faint mark even after cleaning. At that point, further scrubbing risks doing more harm than good.
We would rather see a door at that stage and advise honestly on whether a specialist restoration product will lift the residual mark, than have someone work away at it with increasingly strong products that end up dulling the surrounding finish.
How we assess discolouration during a visit
We look first at where the discolouration sits on the door and cross-reference that against likely causes: near the bottom suggests leaf or splash staining, an even wash across the whole face points to UV fade, and patchy green or black areas usually mean organic growth in a shaded spot. That pattern tells us most of what we need before we even touch the door.
A simple test clean on a small area during the visit usually settles the question on the spot, since genuine staining lifts within a couple of minutes of proper attention, while true pigment change does not shift no matter how long you work at it.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Quarterly clean (DIY) | Under £10 in materials | Prevents most discolouration building up |
| Non-abrasive composite cleaner | £8-£15 per bottle | For stubborn algae or salt haze |
| Professional deep clean and assessment | £45-£75 | Includes a check for coating issues |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Any suspected coating failure.
Prevention
- Wash every 3 months.
- Annual wax.
