How to remove hull stains and waterline scum

The short version: Brown waterline stains are mineral and organic deposits that normal washing does not remove. Acid-based hull cleaners are the usual fix, in liquid or gel form. Protect metal like your trailer, avoid gritty powders like Ajax and Comet, and wax or seal the hull afterward.

On this page: Why washing doesn't work Steps What to avoid Which product Common questions

Why washing doesn't work

The waterline streak, known along the Intracoastal Waterway as the "ICW mustache," survives normal washdowns. Standard detergents remove some dirt but not the brown stain. Practical Sailor found that acids do a good job of removing waterline stains.

Steps

1. Wash the hull first so the cleaner can reach the stain.
2. Wear gloves and eye protection, and protect metal. Acid-based cleaners are strong and can eat metal finishes, so cover or keep it off a galvanized trailer, chrome, and stainless. Follow the label and never mix cleaners.
3. Apply a hull cleaner to the stain. Liquids work fast and are what we reach for most. Gels cling to the hull, so they are easier to control, stay off bottom paint, and have less acid smell.
4. Let it work for the time on the label, then lightly scrub with a soft sponge or brush.
5. Rinse thoroughly.
6. Fix and protect. Acid cleaners leave the gelcoat bare. Fix any oxidation or scratches, then wax or seal it. A protected hull is also easier to clean next time.

What to avoid

• Gritty cleaners. Practical Sailor warns the outer layer of gelcoat can be thin and vulnerable to abrasive cleaners.
• Hardware-store muriatic acid. It is the same acid found in some hull cleaners, but it is sold stronger, eats galvanized and chrome metal on contact, and burns skin. A product made for hull stains is the safer choice.
• Leaving it for months. The stain gets harder to remove over time.

Which product

See hull stain removers on our test list. They are marked Not yet tested until our results are in.

Common questions

Why is my waterline stain brown?

Causes vary with the water where the boat sits, but it is usually mineral deposits and organic buildup that bonded to the gelcoat.

Will this remove oxidation too?

No. Stain removers clean the surface. Oxidation needs a compound. See our oxidation guide.

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