How to Remove Rust Stains From a Fiberglass Boat

Steel has long been prized for its durability, ideal for large ships that require great strength to carry immense loads. But it wasn’t long after steel boats became popular that boat owners soon discovered one fatal flaw - rust. That’s why fiberglass soon entered the picture as a worthy replacement for steel.

Durable, lightweight, and lasting, fiberglass has become a popular choice for smaller vessels. But even these dependable boats can soon develop rust stains. The question now is how to remove rust stains from a fiberglass boat. Fortunately, we’ve got you covered with this step-by-step.

What Causes Rust on a Fiberglass Boat?

First off, it’s important to consider where rust comes from in the first place. The reason why it forms on steel (among other types of metal) is because of the presence of iron. Iron bonds with oxygen atoms in water, creating a layer of iron oxide, which is in simple terms, rust.

Fiberglass shouldn’t technically develop rust or rust stains because it’s a composite material made from glass and resin. Essentially, it doesn’t contain any iron. And where there’s no iron, there shouldn’t be any rust. So why in the world are there rust stains on your boat, you might ask?

Well, all of those hardware fixtures around your vessel are probably made from stainless steel. If you take a closer look, you’ll notice that a lot of the stains start off at the base of these hardware fixtures. And with time, that tricky rust can crawl and drip all over your boat, leaving unsightly stains in its wake.

What You Need to Remove Rust Stains From Fiberglass

extra soft nylon bristles

Before anything else, it’s important to keep in mind that fiberglass can be a delicate material. So while it might be tempting to bust out the hard bristle brushes and bleach, you might want to stick to something a little milder. To prepare for your cleaning project, see to it that you have the following stuff on hand:

Step-by-Step Guide

marine uv protectant spray for dust and dirt repellant

Now that you’ve got everything you need, it’s time to get to cleaning. Rust stains on fiberglass can become particularly stubborn the longer you leave them without proper cleaning. So depending on how long your stains have been around, you may have to escalate your cleaning process as described in these steps:

Remove the Hardware

It’s likely that none of the hardware on your boat is permanently fixed. That means with a screwdriver, wrench, and a little bit of elbow grease, you should be able to pop most of them out of place.

This is important since rust stains can start at the base of these fixtures, which means any stains you find on the boat are probably just the stains that managed to drip out from under the hardware. This also gives you a chance to clean the hardware itself and treat it to prevent rust in the future.

Clean with Mild Soap

You’ve probably cleaned your boat in the past, and now would be the perfect time to put that tried and tested strategy into practice. Using your favorite mild soap, give your boat a nice, gentle wash down and see how it affects the rust stains.

For younger stains, this might be all it takes to get those unsightly discolored spots to go away. But if your stains have developed to become more stubborn, it could help to move on to the next step of the process.

Use Baking Soda

For more difficult stains, a good ol’ dose of baking soda might do the trick. Take some baking soda and dust on a generous layer on your problem areas. Then take your soft nylon bristle brush, dip it in some water, and dab the powder around the stains.

The water and powder should form a thick paste that covers the entire stain. Once you’ve got the mix caked on your stains, leave it to work its magic and come back after an hour. You can then take your bucket of water and rinse away the residue to reveal the clean fiberglass underneath.

Try Acetone

Okay, so if the baking soda step just didn’t work out like you had hoped, then maybe you might be able to get rid of those stains with acetone. A word of caution though - acetone is something of a last resort. This stuff can be pretty powerful and may damage your boat if improperly used.

To use acetone, make sure you’re in an open, well-ventilated area since this stuff has been known to irritate to the eyes, nose, and throat, as well as cause headaches, dizziness, and shortness of breath. Take your white rag, add some acetone, and gently rub away at the rust stains. Don’t use strong pressure and make sure you rinse away any acetone residue when you’re done.

Protect with a Proper Spray 

Once those stubborn stains are gone, and before you put back the steel hardware, make sure you protect your fiberglass boat with the proper spray. The 303 brand sells a protectant spray specifically for fiberglass.

Providing a layer of protection against fading and cracking, the formulation also keeps your fiberglass looking polished and brand new. The non-greasy solution provides maximum UV protection and helps repel stains and dirt to keep your boat clean for longer.

We recommend using the 303 Marine UV Protectant Spray.

Tips for Cleaning Rust Stains Off of Fiberglass

There are a bunch of cleaning techniques that might seem like they make a lot of sense, but actually pose a risk to your boat’s integrity. So before you try anything clever, make sure to follow these tips for cleaning rust stains off of your fiberglass boat:

Avoid Abrasion

Some guides might tell you to use a ‘mildly abrasive’ cleaning agent or tool to really get in there. But fiberglass material uses a delicate Gel Coat that’s prone to scratches and scuffing. Using an abrasive cleanser or tool doesn’t only risk damage to your boat, but also increases the potential of developing more stains later on.

Use Distilled Water

Water that comes straight from the tap will contain lots of minerals that cause staining on the rest of your boat. If you’re hoping to get a really polished clean, then you’d best use distilled water that doesn’t pose the threat of forming mineral stains and water marks on your boat’s surface.

Skip the Bleach

We all know bleach as the universal cleaning solution, but not for fiberglass. That harsh chemical can cause red stains on fiberglass, and may even ruin the Gel Coat resin on the exterior of your boat. As a general rule with fiberglass, any and all harsh chemicals should be avoided. Stick to mild soaps and baking soda, especially if your stains are relatively new.

How to Remove Water Stains and Hull Oxidation

marine oxidation remover for fiberglass boats

When fabricating a fiberglass boat, a Gel Coat is typically the first layer of resin applied to the mold. This is essentially the layer of your boat that comes into direct contact with moisture and water. Over time, it can oxidize, lending a hazy, dull, chalk-like appearance to your hull. This can make your fiberglass boat more prone to staining.

The best way to remove water stains and hull oxidation would be with an appropriate remover formulation. Getting some oxidation remover works wonders on fiberglass boats, safely removing oxidation from Gel Coat surfaces. The stuff also provides a brilliant shine that can make your boat look brand new.

Some boat owners on forums have suggested the use of a power washer to get rid of tougher oxidation and water stains. While there’s a lot of anecdotal evidence to support these claims, it helps to err on the side of caution when using something as powerful as a pressure washer on a delicate Gel Coat.

First off, it would be wise to feed your washer distilled water to prevent any future water stains that develop as a result of mineral presence in the moisture. Then, see to it that you use a wide fan spray, and avoid bringing your pressure washer too close to the boat’s surface to control the strength of the pressure.

And of course, as with any cleaning project, it’s important to make sure that you that you seal that baby up with protectant when you’re done to keep it safe from stubborn, hard-to-clean stains in the future.

We highly suggest using Meguiar's M4916 Marine Oxidation Remover.

You may also be interested in painting a fiberglass boat. If so, we have the perfect step by step guide for you: How to Paint a Fiberglass Boat

As Good As New

Fiberglass boats can be tough and lasting, but they require delicate care just the same. To make sure you’re giving your vessel the proper maintenance it needs, it helps to understand the technicalities of how to remove rust stains from a fiberglass boat. Just remember to keep your cleansers and tools as mild as possible, and protect that boat with the right formulation when you’re done so you can stretch the time between now and your next deep cleaning.