{"id":"wpc-maintenance10-article-02-en","slug":"composite-decking-water-marks-tannins","locale":"en","url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/","title":"Water marks on composite decking: limescale, tannins and drying rings","answer":"A water mark can be residue left by drying water rather than permanent colour loss. The first task is to identify whether the mark is mineral deposit, organic staining, cleaner residue or a surface change before applying another chemical.","author":"J. A. Gill","faqIds":["wpc-maintenance10-faq-02-1-en","wpc-maintenance10-faq-02-2-en"],"tagIds":[],"noindex":false,"related":["wpc-maintenance10-article-01-en","wpc-maintenance10-article-04-en","wpc-maintenance10-article-06-en"],"version":1,"modified":"2026-10-17","reviewer":"","sections":[{"id":"section-01","text":"White rings near sprinklers or planters may be mineral deposits. Brown outlines below wet leaves may involve organic extracts. Streaks following washing may be detergent residue. These are clues, not a diagnosis from colour alone. Photograph the dry surface and note water sources, recent building work and previous treatments.","heading":"Read the pattern before treating it","references":[],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#section-01"},{"id":"section-02","text":"Remove loose contamination and wash a small area with the approved mild cleaner. Rinse completely and let it dry before comparing. Where hard water repeatedly leaves spots, trial a suitable low-mineral rinse. Do not assume vinegar, acid descaler or wood brightener is approved for every cap and accessory.","heading":"Clean the least aggressive way first","references":[],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#section-02"},{"id":"section-03","text":"Wood-derived compounds can contribute to early surface marks in some uncapped products. Exposure and washing may change their appearance, but no universal disappearing time applies. Capped products have a different surface; an unexplained stain should not automatically be called tannin bleed. Ask the supplier to identify persistent marks before escalating treatment.","heading":"Tannins and new uncapped boards","references":[],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#section-03"},{"id":"section-04","text":"Redirect irrigation away from boards, use suitable planter feet and keep outlets open. Avoid allowing dirty cleaning water to dry in isolated puddles. A dark wet patch and a dry mark are different observations: persistent trapped water may point to support, drainage or ventilation problems requiring installer attention.","heading":"Prevent the return of drying marks","references":[2],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#section-04"},{"id":"product-instructions","text":"Identify the manufacturer, collection, capped or uncapped surface and installation date before choosing a treatment. A method suitable for one composite can damage another. Start with loose-dirt removal, then a small test using the permitted cleaner and a soft non-metal brush. Rinse thoroughly and assess when completely dry. Follow the product's dilution, contact time and care instructions; never mix cleaning chemicals. Restrict access while the surface is wet or unsafe.","heading":"Product instructions come first","references":[3],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#product-instructions"},{"id":"green-plank-support","text":"For Green Plank decking, fencing or cladding, request the current care instructions for your exact profile and finish. The Green Plank links below are commercial documentation resources. These guides provide general care advice, not a product certification or a decision on any individual warranty claim.","heading":"Green Plank product support","references":[1],"url":"https://woodplasticcomposite.net/guides/composite-decking-water-marks-tannins/#green-plank-support"}],"seoTitle":"Water marks on composite decking: limescale, tannins and drying rings","topicIds":[],"published":"2026-10-17","tableText":"Observation|Recommended response\nWhite rings after drying|Investigate mineral deposits and irrigation\nBrown leaf-shaped marks|Remove leaves and seek approved stain advice\nMarks survive careful cleaning|Document dry photographs and contact the supplier","takeaways":"A water mark can be residue left by drying water rather than permanent colour loss. The first task is to identify whether the mark is mineral deposit, organic staining, cleaner residue or a surface change before applying another chemical.","categoryId":"wpc-maintenance10-category-en","reviewedAt":"","change_note":"","description":"Identify water spots, mineral deposits, tannin marks and drying rings on WPC decking, then choose a careful cleaning approach without damaging the finish.","heroImageId":"","referenceIds":["wpc-maintenance10-ref-1-en","wpc-maintenance10-ref-4-en","wpc-maintenance10-ref-6-en"],"review_status":"","table_caption":"Practical inspection checklist","seoDescription":"Identify water spots, mineral deposits, tannin marks and drying rings on WPC decking, then choose a careful cleaning approach without damaging the finish.","translationGroup":"wpc-maintenance10-v1-article-02","author_id":"j-a-gill","references":[{"url":"https://greenplank.eu/products/composite-decking-and-wpc-boards-eu-made-green-plank/terratuff-capped-composite-decking-eu-made-green-plank","type":"web","title":"Green Plank — Decking product information","noindex":true,"accessed":"2026-10-17","seoTitle":"Green Plank — Decking product information","publisher":"Green Plank","description":"Green Plank — Decking product information","evidenceType":"manufacturer-commercial-resource","seoDescription":"Green Plank — Decking product information"},{"url":"https://greenplank.eu/documents/green-plank-nfc-decking-installation-guide.pdf","type":"report","title":"Green Plank — NFC decking installation guide","noindex":true,"accessed":"2026-10-17","seoTitle":"Green Plank — NFC decking installation guide","publisher":"Green Plank","description":"Green Plank — NFC decking installation guide","evidenceType":"manufacturer-installation-guide","seoDescription":"Green Plank — NFC decking installation guide"},{"url":"https://www.hse.gov.uk/slips/cleaning.htm","type":"web","title":"Health and Safety Executive (HSE) — Cleaning and slip prevention","noindex":true,"accessed":"2026-10-17","seoTitle":"Health and Safety Executive (HSE) — Cleaning and slip prevention","publisher":"Health and Safety Executive (HSE)","description":"Health and Safety Executive (HSE) — Cleaning and slip prevention","evidenceType":"public-safety-guidance","seoDescription":"Health and Safety Executive (HSE) — Cleaning and slip prevention"}],"faqs":[{"question":"Are water marks always a defect?","answer":"No. Deposits may sit on the surface, but persistent marks need identification rather than automatic dismissal."},{"question":"Can I use vinegar?","answer":"Only if the exact product's care instructions approve it, including dilution, contact time and rinsing."}]}