Waterline Stain Removers

We scoured marine and hardware stores and came up with seven options for cleaning waterlines. Aurora, a liquid, was the top performer, but also the most toxic—so keep your gloves and eyewear handy. Two gels, Davis FSR and Y-10, are the runners-up.


Of all the stains a fiberglass hull can sustain—rust, oil and blood, for instance—none, at least in our experience, can be as tough to remove as a boat-length streak along the waterline. Sometimes it’s the only blemish on an otherwise sparkling hull.

Waterline Stain Removers

While these stains can be the result of a variety of factors—algae, petrochemicals and, in fresh or brackish water, tannin—most are the result of metallic salts caused by the interaction of dissolved metals in the water and air. These stains are similar to the brown or green stains that appear in a toilet bowl.

The cause of a stain should, then, determine the choice of hull cleaner. Detergents, such as the ones that are present in most hard-surface cleaners have little or no effect on these waterline stains when used by themselves; acids, however, do a good job of removing them. As with other aspects of boat maintenance, the marine industry has created specialized waterline stain removers that, depending on what causes your streaks, may not be strong enough to do the job or prove to be overkill. It’s always best to keep in mind that what works the fastest and most thoroughly may also be damaging to your gelcoat.

Generally, stains are removed either by surface cleaning—as with detergents and/or chelating agents (chemical compounds that break up others), by etching acids or by abrasion.

Stronger acids or abrasive substances can eventually damage or dull your gelcoat. Acids will dissolve it, while abrasives will remove it. The stain will come off, for sure, but your hull will gradually lose some of its luster—and stain more quickly in the future. You can limit gelcoat damage by avoiding abrasive liquids and pads (which can scratch) and by washing acids off as soon as they’ve done their job. During our test, we carried a spray bottle with a small amount of baking soda dissolved in water to neutralize the harsher products and to avoid gelcoat damage. This was effective.

What We Tested
We obtained five specialized cleaners from different manufacturers at several local chandleries, choosing only those that stated they were suitable for removing waterline stains.

Although descalers might work on dissolved iron stains, we did not include them in the test. Nor did we use any abrasive products or cleaning pads; our test cloths were soft white cotton.

Most cleaning products advise you to wear adequate protective gear, including gloves. Several of the cleaners we looked at contained some fairly strong acid combinations (hydrochloric and phosphoric) that absolutely require gloves. After one session, despite washing our hands, we started feeling a burning sensation on our forearm where some of the acid had dripped down off of our gloves. Our advice: the longer the gloves the better. (The drippings also removed the wood stain from part of our house’s deck.)

Waterline Stain Removers

In addition, work upwind of the spot you’re cleaning because the corrosive fumes generated by some of these cleaners can be harmful. And it’s wise to wear eye protection, either goggles or sunglasses, even when working with milder solutions such as oxalic acid.

What We Found
Before we embarked on cleaning a hull, we tried the solutions on some pieces of red gelcoat to see if there would be any adverse effects. We let the cleaners sit awhile, too. None, including the harshest, actually took pigment off to the extent that we could detect it on our cleaning cloths.

Our test boat was a 1970s Seabreeze that had a fairly clean hull (light blue in color) but a firmly established algae/iron oxide rim along its waterline. In this test, all of the cleaners worked fairly well, although some worked faster than others. A few tough spots required several applications with one of the stronger acids.

Aurora Waterline Stain Remover ($9.95/16.5 fl. oz.)
The active ingredient here is hydrogen chloride (essentially hydrochloric acid), which is fairly potent, creating corrosive fumes that should be avoided (the label carries this warning).

The product worked well, but the fumes were bothersome. You apply it with a chemical-resistant paint roller, leave it for 20 to 30 minutes and rinse off.

FSR Fiberglass Stain Remover ($9.95/16 fl. oz.)
Produced by Davis Instruments, this dark-blue, gel-like product relies on relatively mild oxalic acid (a bleaching agent) as its active ingredient. Directions are to shake well, apply with cloth, brush or sponge, and wash off after several minutes. FSR worked reasonably well on most of the stains but required a bit more scrubbing. The gel made it easier to keep in place on a vertical surface.

Y-10 ($8.95/12 fl. oz.)
This was our favorite moderate-duty cleaner, similar to the FSR gel. It has an oxalic acid base. Based on our test results, we thought that Y-10 produced slightly better results than the FSR (it’s a tad thicker) and was just as easy to work with.

Developer Robert Oppenheimer said oxalic acid has “an affinity for iron,” converting it to iron oxalate, which washes away. We liked the product and its results as a whole. We would choose Y-10 over the other runnier liquids.

MaryKate On & Off ($10.50/32 fl. oz.)
This liquid “hull and bottom cleaner” seemed to be the most potent of all, with an active ingredients list that includes hydrochloric, phosphoric and oxalic acids. The label clearly, and properly, warns that the cleaner is harmful to skin and eyes and cautions the user to “not breathe the vapors or fumes.” But that’s not easy when you are working close to the hull, scrubbing off stains. Directions say to apply full-strength with a natural-bristle brush and rinse with fresh water.

Like Aurora, this product definitely works on the most stubborn waterline stains, but you have to deal with the fumes.

West Marine Hull Cleaner ($6.99/32 fl. oz.)
Made for West Marine by StarBrite, we found this greenish detergent-type liquid with phosphoric acid fairly mild to use (and breathe) and capable of removing various scum and green algae stains. We felt comfortable using it to clean much of our test boat. But it was not completely effective on the toughest, ground-in stains. That’s the tradeoff with a milder solution.

Jeff Tieger of StarBrite said that the product also contains chelating agents for breaking up insoluble materials, such as those found in rust stains, a wetting agent to help penetrate scum, and a detergent component (a surfactant) to help break through the dirt.

The label cautions against letting the cleaner run onto bottom paints, waxed surfaces or galvanized trailers, a caveat that would apply to most of the cleaners included in this test.

Cleaning the waterline is close-up work, so we’re inclined to avoid the stronger acid solutions (especially those with hydrochloric acid) in favor of the milder cleaners. Of these, we preferred the Y-10, which clings to surfaces regardless of angle and does the job on most stains. We’d use one of the stronger cleaners only in the case of especially stubborn stains.


Contacts— Aurora Marine, 7015 Ordan Dr., Mississauga, Ontario, Canada L5T 1Y2; 905/564-4995. FSR, Davis Instruments, 3465 Diablo Ave., Hayward, CA 94545; 510/732-9229. MaryKate, Marikate Ship Shape Inc., PO Box 150, Bohemia, NY 11716; 800/227-8307. West Marine, 500 Westridge Dr., Watsonville, CA 95076; 800/262-8464. Y-10, Propco Marine Associates, 16 Mortimer Dr., Greenwich, CT 06870; 203/637-9515.

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at darrellnicholson.com.