Which gelcoat cleaners are best at combating waterline yellowing on boat hulls? Ben Meakins dons his rubber gloves and puts 10 products to the test

Gelcoat is an amazing material – hard-wearing, easily repairable and eminently restorable – but as it gets older it becomes porous and starts attracting stains. Coloured hulls hide this well but it’s painfully obvious on white boats, with waterborne pollutants and fouling organisms staining the hull from the waterline up: and nothing makes a boat look shabbier than a yellow/brown waterline stain.

Luckily for us, there are few more satisfying jobs than removing such stains when you use the right tools. Even better, for once in boat ownership it doesn’t require much elbow grease. Instead, a chemical reaction does the job for you. Chandlery shelves groan under the weight of treatments. Most contain oxalic acid, with some supplied as a liquid and others mixed with a gel to help them stay in contact with the hull as long as possible. There are other acids which will remove the staining, and some people even claim that lemon juice, if left long enough, will help.

However, not all acids will leave the gelcoat untouched, so it’s worth using products that are certified safe for boats. Have a look at the label –although oxalic acid is the most common active ingredient, citric acid and phosphoric acid are used in some products.

Online advice suggests buying a tub of oxalic acid powder and mixing it with wallpaper paste to make your own treatment, but the potential for health issues means we’ve limited this test to products commercially available in the UK. As with any chemical reaction, the results will improve the warmer the temperature (as long as it doesn’t dry out) – and Y10, for instance, claims improved results above 10°C. Our test day in February had colder temperatures of around 6-8°C, so we left the products on the hulls for the longest time specified on the containers.

How we tested them


We borrowed Barney Smith’s Impala 28, Imptish, which, after a busy season moored on the Hamble River had a heavy brown waterline stain along her 30-year-old white gelcoat. With the boat out of the water at Deacons Boatyard, we cleaned off the hull with fresh water before drying it off and taping out a test panel on the starboard side, with strips between left untreated to show the hull’s condition before the test. We applied each product to its panel according to the instructions on the bottle, leaving it on for the maximum time specified before rinsing off and comparing the result with the surrounding areas.

  1. 1. Introduction
  2. 2. International Stain Remover
  3. 3. Silky Marine Bright
  4. 4. Starbrite Hull Cleaner
  5. 5. Farecla Professional Rapid Stain Remover
  6. 6. Y10 Stain Removing Gel
  7. 7. Meguiars Marine Heavy Duty Oxidation Remover
  8. 8. Grunt! Boat Cleaner
  9. 9. Chine Shine
  10. 10. Boat Buddy Marine Surface Wash
  11. 11. Hempel Cleaning Gel
  12. 12. PBO Verdict
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  • janefire

    thanksfor your reply, nick, i think rhibarb is interesting. nontheless i gues there is a difference between seawater and chemicals that destroy the seawater envonronment. actually i think barkeepers friend is made form rhubarb, too 😉

  • Hi Jane. Silky Bright is made from oxalic acid which is a natural product. Oxalic acid is used by bee-keepers to disinfect hives. It is safe for bees and is biodegradable as it is made from rhubarb. Silky Marine’s parent company makes a vast amount of product for the UK water industry which is highly regulated. Silky make enzymes that you ad to toilet waste holding tanks to break down waste. However, enzymes are chemicals and all natural compounds and naturally found liquids are chemicals. Adding natural elements together to form new compounds is chemistry. Seawater is a chemical.

  • janefire

    great article with realistic hull-environment to test on. but what about more nature friendly test rounds? i think the quest of the future will actually be how to use less polluting materials. i just fell over “barkeeper’s friend” recently as a very clean solution in both ways – hull is clean and nature stays clean, too… but i am no expert, have you ever tried testing organic methods (like natural ingredients or enzymes) AGAINST chemical methods in a comparison? i would be very interested in a pioneery trip to that frontier, as natural solutions become more and more relevant, also to the industry, right now.