Mildew

Mildew is a form of fungus closely related to mould, which is known for its distinctive odour.

Convertible roofs are especially prone to leaks, and once moisture enters your car it provides a good environment for mould and mildew.

Mildew needs minimal organic matter to grow on, but it does need dampness, so it is important to prevent water ingress by properly maintaining your soft top's drainage system and rubbers. 

The smell of mildew and stale water can linger, and may need professional odour treatment.

What it means

Mildew is a form of fungal growth that appears where a car’s interior has been slightly damp for a while. It often shows as light grey, white or pale-coloured fuzzy or powdery patches on seatbelts, seat fabrics, plastics, rubbers and headlinings. It commonly follows a smaller water leak, persistent condensation or storing the car damp with poor ventilation. A musty, stale smell usually accompanies even light mildew, sometimes before the growth is obvious.

Why it matters

• Early warning of a damp problem: Mildew is usually the first visible sign that an area has been staying damp - often from a minor leak, wet carpets or constant condensation.
• Unpleasant to sit in: The musty odour and powdery deposits make the car feel dirty and uncared for, and are not pleasant for occupants to breathe.
• Can progress to heavier mould: If the damp is not dealt with, light mildew can turn into heavier, darker mould growth that penetrates deeper into fabrics and foams.
• Leaves marks and staining: Even once killed off, mildew can leave visible stains on fabrics, belts and trims that need proper cleaning or, in bad cases, replacement.

Where you’ll see it

You will see mildew mentioned on leak reports, interior inspection sheets and valeting or decontamination quotes. Typical comments include light mildew on seatbelts, mildew on seat backs, mildew on plastics or mildew present on headlining. It is especially common on cars that have had damp carpets, minor sunroof or door leaks, or have been stored for long periods with wet interiors and closed windows.

Context

Mildew sits in the same family as mould, fungus and other biological contamination, but is often a lighter, earlier stage. It is still a sign that conditions inside the car have been suitable for fungal growth - warmth, moisture and stagnant air. A proper cure involves more than just wiping off what you can see. The usual sequence is to find and fix the water leak or damp source, dry the interior properly, and then clean and treat affected surfaces to remove visible growth and reduce odour. If soft furnishings, underlay or soundproofing have been wet for longer, the issue is likely to be classed as mould rather than just mildew and may need a more intensive decontamination and drying process.

Common mistakes

• Wiping away visible mildew but leaving the underlying damp problem - such as a leak or soaked underlay - untouched.
• Trying to mask the musty smell with air-fresheners rather than dealing with the contamination and moisture.
• Cleaning only the obvious patches and ignoring seatbelt webbing, seat bases and hidden plastics where mildew also thrives.
• Leaving a slightly damp car shut in warm conditions, allowing light mildew to develop into more serious mould contamination.

Written by Danny Argent. Last updated 03/12/2025 18:38