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 boat 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.

Read YBW’s guide to the best hull cleaners on the market

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 the gelcoat cleaners

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.