Addressing Hull Stains: Temporary and Permanent Solutions
February 20, 2024 7 min read
I’ll never forget that moment when I first noticed it on my first boat. Like a frog caught in a slow boil, I hadn’t noticed the discoloration as it slowly took hold, but that day, it hit me hard. I’d only owned that boat for a few months, but it was my pride and joy — one of those boats that, even to this day, I think about. Eighteen feet, tunnel hulled and great lines, that flats boat represented a major upgrade over anything I’d ever owned before. Now, I was watching a brown stain taking over the baby blue gelcoat, and even after some determined scrubbing, it wasn’t going away.
Over the next few months, I tried different soaps, bleaches, sponges and rags, but out of fear of doing more damage to the gelcoat than good, I decided to live with it until I could figure it out (there wasn’t any “Google” back in those days). An unexpected move to the mountains of Virginia prematurely ended my ownership of that boat, and my next boat was a jet-driven aluminum boat designed to bounce up rocky rivers.
Since hull staining was the least of its cosmetic problems, I forgot about the problem and enjoyed a blissful ignorance for several years. It wasn’t until I finally returned to the salt that I had to face the issue again. Within a few months of my move to coastal North Carolina, the tannic water did its trick, and the gleamingly white hull of my two-toned Maverick was being attacked by an ugly brown hue at the waterline.
How NOT to Remove Hull Stains
Hull staining is common in both fresh and saltwater environments, but it happens most rapidly in stained waters. Watch the boats backing in at the local boat ramp, and it becomes painfully obvious who knows how to deal with this issue and who doesn’t. Once you understand the cause and the solution, seeing a nice boat with a stained hull is a source of amusement. It’s just such a simple thing to address once you know what to do about it.
The stain is embedded into microscopic holes in your gelcoat, and it can’t be wiped away. Using abrasive sponges or rags will make the situation worse.
Let’s start with what NOT to do. That “stain” is a true stain and it has to be addressed accordingly. It is embedded into microscopic holes in your gelcoat, and it can’t be wiped away. Using abrasive sponges or rags will actually make the situation worse. Chemicals can work, but many can do significant damage to the hull, trailer or metal components on the boat, so care must be taken.
Treat the Symptoms or Fix the Problem?
There are two ways to address the issue. You can fix the fundamental problem, or you can treat the symptoms. Both options have benefits, and in some situations, “addressing the fundamental problem” will still require “treating the symptoms” periodically. Before we get into these two options, it’s important to understand the cause of the stain — and it’s more than just murky water.
What Causes Hull Staining?
When a boat is built, the gel coat is sprayed against a highly polished mold, and after further buffing, the gel coat is virtually impervious to stains. As gelcoat ages, however, it can oxidize and become increasingly porous. Hulls that are infrequently waxed will oxidize more quickly than those that are regularly protected. Once the oxidation process begins, stains seep into the porous gelcoat, discoloring the hull.
Quick Fix: Acid-Based Hull Cleaners
The good news is that, for short term fixes, there are dozens of products that can easily remove these stains, most without any scrubbing. There are a number products that are specifically marketed for boat owners, most of which work exceptionally well at removing hull stains.
The good news is that, for short term fixes, there are dozens of products that can easily remove these stains, most without any scrubbing.
I have been using StarBrite Fiberglass Ultimate Stain Remover, which sprays on, and within minutes the stains disappear. Depending on the situation, I may use a brush to spread it around, then I simply let it sit for a few minutes and rinse it away. Another product that works for these stains is Davis FSR 790, which comes in a gel and can be applied more strategically.
Both of these products use oxalic acid, which will not damage your gelcoat, but can damage other materials. While oxalic acid is often used to clean and polish some metals, repeated or constant exposure can cause etching. If you are washing your boat on a trailer, it is recommended that you ensure the trailer is wetted down to dilute any acid that lands on it, and to rinse it off thoroughly. I do this, and have not had any issues with the products damaging my trailer.
The Last Resort
Other less expensive products can address the stains as well. Barkeeper’s Friend is a popular choice, and like the aforementioned products, contains oxalic acid. However, it also contains mild abrasives, especially the powdered form of the product, and in the long run this can actually exacerbate your hull staining issue.
If oxalic acid cleaners aren’t strong enough to remove your stains, the last resort is a product containing muriatic or hydrochloric acid.
If oxalic acid cleaners aren’t strong enough to remove your stains, the last resort is a product containing muriatic or hydrochloric acid. StarBrite’s EZ-ON EZ-OFF Hull & Bottom Cleaner and MaryKate’s On & Off Hull and Bottom Cleaner are marketed specifically for marine applications; The Works Toilet Bowl Cleaner contains a similar concentration of acid. All are highly corrosive. While a quick cleaning won’t harm gelcoat, they can do significant damage to other materials, especially metals, and should be handled with extreme caution.
Addressing the Root Cause: Restoring Gelcoat
While fiberglass stain removers provide an easy, temporary solution to staining, the recurrence of hull stains is a symptom to a deeper issue, and one that should eventually be addressed. As mentioned before, “healthy” gelcoat should be impervious, and will resist stains. If your hull stains quickly and repeatedly, your gelcoat has become oxidized. Left unaddressed, this process will accelerate, requiring more and more severe countermeasures to correct the issue.
The long-term solution to frequent hull staining is to get your gelcoat back to its impervious state.
The long-term solution to frequent hull staining is to get your gelcoat back to its impervious state. That means polishing away all of the oxidized, porous surface material and getting back to a smooth, impervious surface. Not only will this bring back the shine to your boat, it will massively reduce the frequency of staining.
Revitalizing Your Boat's Shine
I’ve done this on the last two Mavericks I’ve owned and have been amazed at the difference. Fishing the tannic stained rivers of coastal North Carolina, I had gotten used to burning through bottles of oxalic acid-based sprays, using them after every trip. Now, after addressing the root cause and polishing my hull, I only use them periodically. I suspect that, with a professional detailer involved, I could reduce the need for stain removers even further.
If the oxidation is minor, using a cleaner wax and dual-action polisher is a relatively simple, if time-consuming, option for DIYers.
Having a professional detailer polish an oxidized boat can be expensive, especially if the gelcoat has deteriorated significantly. In some cases, restoring severely oxidized gelcoat requires multiple steps, starting with a fine-grit wet sanding and progressing down through various compounds and polishes. In other cases, a simple one- or two-step process can remove the surface oxidation and get the boat back to near perfect shape. If the oxidation is minor, using a cleaner wax and dual-action polisher is a relatively simple, if time-consuming, option for DIYers.
Sealing Gelcoat to Prevent Staining
Once oxidation is removed, gelcoat should be sealed with a quality marine wax or ceramic coat to prevent future oxidation. Be careful about the wax you choose for your boat; some waxes will yellow with prolonged UV exposure. “Cleaner waxes” contain mild abrasives and should only be used if oxidation is present.
Having your hull polished will not only make your boat look new again, it will help it resist stains and protect the gelcoat for many years to come.
If your boat has a stained hull, the good news is that, with virtually no elbow grease and 15 minutes, you can make it disappear, at least for a little while. However, when you see that stain repeatedly, think of it as your boat’s way of asking for a little TLC. Spending the time — or the money — to have your hull polished will not only make your boat look new again, it will help it resist stains and protect the gelcoat for many years to come. Once you understand the cause and solution for stains, you will likely find yourself shaking your head next time you see a badly stained hull show up at the ramp.
