How to Clean a Fiberglass Boat Hull 

When it comes to cleaning fiberglass boat hulls, there are plenty of ideas floating around out there as to how to do it the right way. Browse the boating forums and you’ll see boat owners extolling the virtues of oven cleaner, toilet bowl bleach and even lemon juice. 

While there may be some merit to these home remedies, you’ll get much better results using products specifically formulated for cleaning boat hulls. There’s no shortage of boat hull cleaning products available, and you should stick to specialized products that have been proven to work.  

Avoid Using Muriatic Acid 

Before we go any further, let’s mention muriatic acid, which is a variant of hydrochloric acid or HCl. Muriatic acid is extremely dangerous, and should only be used by professionals. 

Many claim HCI is the only way to go when cleaning a fiberglass boat hull, because they heard it's what the pros use. However, an inexperienced person using HCI is not only reckless, but risks badly damaging their boat, trailer and health. Muriatic acid is highly caustic, and can easily cause irreversible damage to skin, lungs and eyes. It’s not recommended for use by any non-professional. 

Cleaning Waterline Stains

Boats used in fresh water gradually develop a yellow or brown stain at and below the waterline caused by tannins, which are mild, brown-colored acids created by plants, trees, algae and soil decaying in the water. 

The best way to prevent freshwater waterline stains is to regularly clean and wax the hull, but if a waterline stain does appear it’s simple enough to remove with boat hull cleaner. To remove light waterline stains, apply boat hull cleaner to the affected areas and leave it in for a few minutes before rinsing it off. Moderate stains may require two or more applications worked into the hull with a brush or rag to get it into any tiny pores in the gelcoat, where stubborn stains are hiding.

Oxalic acid or phosphoric acid is usually the active ingredient in hull cleaner. Both are much less corrosive than HCI. Some hull cleaners even contain mild citric acid found in lemons. Of course, any acid could be harmful to your hull’s gelcoat, so be sure the product is safe on fiberglass/gelcoat, and never expose the gelcoat to the cleaner for too long. Always follow the product manufacturer’s directions. When choosing a hull cleaner, note whether it’s a gel or a liquid. Gel clings to the hull better, so it keeps the acid in contact with the stain longer, making it more effective at stain removal.

When choosing a hull cleaner, make sure it won’t damage the metal, rubber, polyurethane or paintwork on your trailer. Also, cover your trailer with plastic sheeting before you clean the hull to ensure the acid in the cleaner won’t damage it. Always wear protective gear when using any boat cleaner.

Cleaning Salt Stains

Boats used in salt water are prone to accumulating salt deposits at and below the waterline. These salt deposits trap dirt that stain and discolor the hull. 

As with freshwater stains, the best way to prevent saltwater stains is to regularly wash and wax the hull. If salt stains build up, treat them with salt-removing products. Salt removers are usually sprayed on and rinsed off, making them easy to use. However, they shouldn’t be applied with a rag or brush, as any rubbing or brushing motion could cause the salt and dirt to become dislodged and act like sandpaper on the gelcoat. Always wear protective gear when using salt removers.

Cleaning Rust Stains

Stains in the gelcoat caused by rust and corrosion from metal fixtures can be treated with hull cleaner. 

However, most people prefer an inexpensive calcium, lime and rust remover. These products are generally less caustic than hull cleaner, so they’re safer to use for the gelcoat, and around your trailer and yourself, but you should still wear protective gear when using these products. 

Cleaning Barnacles and Barnacle Husks

Barnacles are the toughest things to remove from a hull. No matter how few, if you find any barnacles, remove them immediately from the hull before they can multiply. 

Special care is needed not to damage the gelcoat when removing barnacles. Only use wooden or plastic implements to scrape away barnacles, such as a plastic paint scraper or a wooden spatula. Once a barnacle is removed, it leaves behind a calcium ring known as a husk. These can be removed with calcium, lime and rust remover, although several applications and additional scraping may be required. Another method is to sand the husk away with fine grit wet and dry paper. However, this scratches the gelcoat and requires it to be polished after sanding.

After Cleaning the Hull

Having cleaned the hull of water stains, salt, rust or marine growth, it should then be washed, dried and waxed before the boat is used again. 

Boat wax provides a protective layer over the gelcoat that helps prevent against future stains or marine growth. The more frequently you wax your boat and the more layers of wax you apply each time, the better protected your boat will be and the easier it is to remove any stains or growth in the future.