9 Tips For Using Muriatic Acid to Clean a Fiberglass Hull

Most fiberglass hull cleaners use oxalic, phosphoric or citric acid to remove stains and barnacle husks from the underside of a boat.

Muriatic acid boat hull cleaning

But sometimes the barnacle buildup is so severe that a stronger acid is needed, which is where muriatic acid comes into play.

What is Muriatic Acid?

Muriatic acid is type of Hydrochloric Acid (chemical name HCI), and is extremely caustic and corrosive. It can easily burn through some metals, plastics, and your skin.

Muriatic Acid hull cleaning tips

Muriatic acid can cause irreversible damage to your skin, lungs and eyes, and even the smallest contact can result in permanent scarring, respiratory problems and blindness. Therefore, muriatic acid should be viewed as a last resort after less dangerous chemicals such as boat hull cleaner or calcium, lime and rust remover have been tried. It’s not recommended that muriatic acid be used by anyone other than a professional. However, if you feel you’ve exhausted every alternative and have to use HCI, here are some tips for handling and using muriatic acid as safely as possible.

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Personal Safety Measures

Tip #1. Wear a faceshield when using muriatic acid. Safety goggles or (don’t even think about it) sunglasses alone are nowhere near enough protection for using muriatic acid. You’ll also need a respirator capable of filtering acid gasses, and a pair of acid-proof gloves. Disposable coveralls are also recommended to protect your clothes. 

Muriatic acid handling protective gear

Tip #2. Have an acid-neutralizing alkaline spray readily available in case of accidental spills or bodily contact with the acid. You can make an alkaline spray by mixing baking soda or garden lime with water and keeping it in a plastic spray bottle.

Tip #3. Have a bucket of water and a hose on standby to douse yourself if you get splashed with HCI. You can add baking soda or garden lime to the bucket of water to make it more effective against the acid.

Protect Your Boat's Trailer

If you’re using muriatic acid while your boat is on its trailer, you’ll need to first protect the trailer because HCI dissolves many metals and plastics. This includes the galvanized steel used to make your trailer and any nylon, polyurethane or rubber rollers and bunk guides or slides. Muriatic acid will also corrode a trailer’s paint or powder coat.

Tips for using muriatic acid for hull cleaning trailer

Tip #4. Cover the trailer in disposable plastic sheeting before using any acid. Be aware that the HCI will corrode the plastic sheeting, so use several layers and be prepared to throw down additional sheets if necessary. It’s better to use cheap disposable sheets than more expensive plastic tarps, because whatever you use will need to be discarded afterwards.

Protect Your Boat

HCl can be harmful to gelcoat, so never use it in concentrated form. Instead, always dilute it with water before being applying it to a boat.

Hull cleaning with muriatic acid tips

Tip #5. To prevent unwanted damage to the fiberglass, don’t expose any gelcoat surfaces to muriatic acid for too long. Apply the acid to small areas, and rinse it off as soon as possible.

Handling Muriatic Acid

Tip #6. Use extreme caution at all times when handling muriatic acid. Store it in a cool, dry place and always keep it in its original container.

Tip #7. When you’re ready to use HCI, you’ll need to dilute it with water in a glass container or acid-proof plastic container. Never use a metal or non-acid-proof container, as the muriatic acid will eat its way through the container.

Muriatic acid HCl for hull cleaning tips

Tip #8. When diluting muriatic acid, always put the water into the container first and the acid last. Putting the acid before the water creates an exothermic reaction that can cause the mixture to splash violently.

Muriatic acid for hull cleaning tips disposal

Tip #9. Dispose of muriatic acid responsibly. Never pour it down a drain, and ask your local municipal waste disposal department for the regulations regarding acid disposal in your town or city.


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