Quick answer
Composite door stains from bird droppings, tannins, sap or wine usually lift completely if treated promptly with warm soapy water or a baking soda paste. GRP is fairly stain-resistant by nature, so most marks are a cleaning job rather than permanent damage, provided they're not left to bake into the surface for weeks.
Overview
GRP is fairly stain-resistant if cleaned promptly. Bird droppings and tannin marks are the trickiest.
Symptoms
- Localised marks that won't come off with water.
Common causes
- Bird droppings.
- Tannins from wet leaves.
- Sap.
- Rust runs from hardware.
Why prompt action matters more than the product you use
GRP's smooth gel-coat surface doesn't absorb liquids the way timber or masonry does, which is why most stains sit on the surface rather than soaking in, and why they're generally removable if you get to them within a reasonable time. The problem is that UV exposure and repeated wetting and drying cycles gradually bake a fresh stain into something more stubborn.
A bird dropping wiped off within a day or two comes away with plain soapy water almost every time. The same dropping left through a warm, sunny week can etch a faint mark that needs a proper cleaner and some patience to fully shift. Speed matters more than which product you reach for.
Bird droppings: the most common and trickiest stain
Bird droppings are acidic and, if left in direct sun, that acidity combined with heat can cause a genuine chemical interaction with the gel-coat's surface finish, not just a surface deposit. This is why they're consistently the hardest of the common stains to shift once they've had a few warm days to sit.
Treat with warm soapy water first, using a soft cloth rather than scrubbing hard, which can spread the stain rather than lift it. If a faint mark remains once dry, a non-abrasive composite door cleaner applied and left for a few minutes before wiping usually finishes the job. Doors positioned under a favoured perching spot, overhead wires or a nearby tree are worth checking more frequently than the rest of the property.
Tannin stains from wet leaves and organic debris
Tannin marks come from decomposing leaves, twigs or other plant matter sitting wet against the door, usually where wind has blown debris into a porch or against a threshold. They show as a brownish, slightly yellow-tinged stain that can look worse than it is.
A baking soda paste, made up with a small amount of water to a thick consistency, applied to the stain and left for ten to fifteen minutes before gentle wiping, handles most tannin marks effectively. It's a mild enough treatment not to risk the finish, but with enough gentle abrasive action to lift a stain that soapy water alone often can't fully shift.
Sap and rust runs
Sap from overhanging trees is sticky rather than chemically stubborn, and usually responds to warm soapy water with a bit of extra dwell time to soften it before wiping. If it's hardened, a small amount of white spirit on a cloth (not soaked, just dampened) can help, followed by a proper soapy rinse afterwards to remove any residue.
Rust runs, usually from a nearby metal fitting, bracket or an old hinge starting to corrode, show as an orange-brown streak below the source. Baking soda paste again works well here, but the real fix is addressing whatever's actually rusting nearby, since the stain will simply return otherwise.
- Sap: warm soapy water, extra dwell time, or a dampened cloth with white spirit for hardened sap.
- Rust runs: baking soda paste, then address the source of the rust.
- Wine or food spills: warm soapy water immediately, blot rather than rub.
What not to use on stains
Bleach is the biggest one to avoid entirely; it's tempting for tough organic stains but it can damage the gel-coat finish and degrade nearby rubber seals with repeated use, even if it seems to work on the stain itself in the short term. Similarly avoid any household cream cleaner with visible abrasive grains, which can create fine scratching that catches dirt more readily afterwards.
Magic sponges, the melamine foam type, are mildly abrasive at a microscopic level and are best used sparingly rather than as a routine cleaner, since repeated use can gradually dull a satin finish over time even though each individual use seems harmless.
When a stain won't lift after two proper attempts
If you've tried warm soapy water and then a proper composite cleaner or baking soda paste, given each a fair go with appropriate dwell time, and the mark is still visible, it's worth having it looked at rather than escalating to harsher products on your own. Some marks, particularly old bird droppings that have had a UV etching effect, genuinely won't come out and pushing harder risks the finish rather than the stain.
We can assess whether a stubborn mark is a stain that needs a specific professional-grade treatment, or an early sign of finish damage that cleaning was never going to fix.
Preventing stains building up on a coastal or let property
Monthly washing, rather than waiting for a visible stain to prompt action, is the single biggest factor in why some doors stay pristine for years while others accumulate a patchwork of marks. This matters more under bird perches, overhanging trees, or anywhere debris naturally collects against the door.
On a holiday let, building a quick check into the changeover routine catches fresh stains while they're still easy, rather than leaving them for the next scheduled deep clean weeks later.
What a stain removal attempt does and does not risk under the guarantee
Using a manufacturer-approved cleaning method to remove a surface stain does not affect the colour-fastness guarantee on a genuine GRP door, since the gel-coat is designed to be cleaned repeatedly over its lifespan. What can affect it is using an unapproved abrasive or solvent-based product that visibly dulls or damages the surface, since that kind of damage is then classed as accidental rather than a material fault.
If you are ever unsure whether a stubborn stain justifies a stronger product than usual, it is worth checking with us first rather than experimenting, particularly on a door that is still well within its guarantee period.
A seasonal stain calendar for Cornish doors
Late spring and early summer bring the heaviest pollen staining as gardens and hedgerows across Cornwall come into full bloom, often leaving a fine yellow dust that turns into a stubborn film if the first rain of the season is followed by strong sun before anyone wipes it off. Midsummer brings sun cream, barbecue smoke residue and general handling marks from a busier household or holiday let turnover.
Autumn's main issue is tannin staining from fallen leaves and general organic debris blown against the door in the first autumn storms, while winter brings road salt and grit thrown up from passing traffic on coastal lanes, plus the occasional oil or fuel splash near garages with an adjoining composite door.
- Late spring: pollen film, wipe before the first strong sun sets it.
- Summer: sun cream, smoke residue and handling marks.
- Autumn: tannin staining from leaves and storm debris.
- Winter: road salt, grit and occasional oil splashes.
Products to avoid when tackling a stubborn stain
Avoid bleach, oven cleaner and other strongly alkaline household products, since these can react with a gel-coat finish and leave a permanently duller patch that stands out more than the original stain. White spirit and other solvents carry a similar risk, particularly on darker colours, where even a brief contact can leave a visible bloom in the finish.
Magic eraser-style melamine sponges are also worth avoiding on a composite door, since they work by mild abrasion, which is exactly what you do not want on a smooth gel-coat surface, however effective they are on kitchen worktops.
Holiday-let stain management between guest changeovers
Rental properties see a wider variety of staining causes than an owner-occupied home purely through the volume and variety of guests, from suncream and barbecue marks in summer to muddy dog paw prints and wet coat contact in winter. Building a two-minute stain check into every changeover, alongside the usual cleaning checklist, catches marks while they are still fresh and easy to lift.
Keeping a small kit of warm-water-safe cleaning cloths and a mild approved cleaner in the property's utility cupboard means a cleaning team can deal with marks immediately rather than leaving them to set in before the next scheduled deep clean.
When a stain is beyond cleaning and needs professional attention
A stain that has been left to set for an extended period, particularly oil, fuel or strong dye-based marks, can sometimes penetrate the microscopic texture of a gel-coat finish deeply enough that ordinary washing will lift most but not all of it, leaving a faint shadow. At that point a specialist restoration product applied correctly is usually still worth trying before assuming the mark is permanent.
We assess these on site with a test clean on a small area, and give an honest view on whether further treatment will fully lift the mark or whether it has become a permanent, if faint, feature of that panel.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Baking soda paste treatment | Under £5 | For tannins and rust runs, self-applied |
| Non-abrasive composite cleaner | £8-£15 | For stubborn bird dropping and general stains |
| Professional stain assessment and treatment | £45-£75 | For marks that won't lift after two attempts |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Stains that won't lift after two attempts.
Prevention
- Wash monthly, especially under bird perches.
