Stubborn stains require serious cleaning strategies, but always in a way that protects the boat.
By Peter Frederiksen | April 8, 2025
Tough Love Protect your boat from stains and damage with expert tips on hull cleaning, teak care, and maintenance from a seasoned Intracoastal Waterway captain.
As a former professional captain, I spent thousands of hours delivering boats along the Intracoastal Waterway. The one constant chore during those many ICW miles was cleaning up the yellow-mustache hull stains that expanded upward from the waterline.
No matter how well a boat is maintained, stains are a constant peril. They are best addressed sooner rather than later. Bird droppings, rust, corrosion, acid rain, mildew, diesel soot, poor quality wash-down water, marine growths and countless more environmental offenders can require intensive cleanings and restorations.
Worse yet, a simple stain that gets ignored can reveal a more involved problem. A rust trail bleeding from a wooden hull once alerted me that the fasteners beneath the paint were rotting. On any modern boat, the number of holes drilled into the transom is astounding: outboard engine mounts, inboard engine exhaust drains, bilge pump discharge outlets, motor well, livewell and cockpit drains, transducers, underwater lights, trim tabs and more. If improperly bedded, each hole is a risk. Caulks and sealants age, and adhesive sealants can lose their grip over time, letting water in.
Stubborn stains that have migrated into gelcoat pores usually respond favorably to “on and off” liquid hull cleaners that remove waterline stain, algae, rust and engine exhaust residue. (These products are not designed for use on painted surfaces.) Care is necessary when using such acid-based cleaners, especially from the hull’s sheer to the waterline. Goggles, gloves and protective clothing are mandatory for safety. A vigorous water rinse completes the task, but sometimes a second application may be necessary. The drill follows with a soapy wash and rinse, and the need to re-wax, because the cleaners leave nothing behind to protect the gelcoat.
Test a small area of the cleaned gelcoat with wax before proceeding. If the wax resists being easy to remove by hand, then apply a mild cleaner wax followed by a few coats of liquid or paste wax. This should fully seal the pores from exposure to the marine elements. Midseason waxings also will help maintain and protect the gelcoat.
Whatever product you use to protect your boat will only be as good as the condition of the surface to which it is applied. A trick I use that helps keep the boat clean is to hose the exterior before leaving the dock. This removes residual grime and dust that collects while the boat is in the slip. The rinse flushes the grit completely, rather than have it slowly drip overboard with the salt spray encountered underway. Later, back at the dock, a gentle soaping with a neutral, phosphate-free detergent will do a good cleaning job without harming the wax that protects the gelcoat.
Even with routine washings, after it rains you may notice dark streaks in vertical areas. No need for alarm: This is grime that has tracked into the wax. It generally wipes off with a wet chamois or a light soaping and rinse.
Topside metal fittings typically clean up with a dab of metal polish. Buff it with fine bronze wool or a nylon scrub pad. Another option is a liquid rust remover such as Whink or Goof Off, or a gel product like Davis FSR. If the stain reappears, it could mean a problem with the quality of the fastener or fitting, or the lack of sufficient sealant. If the fastener screws into a wood core, the wood can be wet.
Prevention with surfaces beyond gelcoat is also key. A good example is cleaning exterior teak properly.
Teak is a dense wood with soft and hard grain. Too much scrubbing with gritty cleansers removes soft grain, leaving an uneven surface. Liquid teak cleaners are generally based around a two-part program. Step one chemically draws dirt and grime from the wood grain with a soft brush or scrub pad. Rinse before proceeding to step two, which brightens the wet wood. After another freshwater rinse, the teak dries into a like-new golden tone.
However, the impurities freed from the grain can discolor and stain the adjacent fiberglass and painted surfaces along the hullsides and transom. It is imperative to flush any surfaces below the teak with generous amounts of fresh water before, while and after applying the teak cleaners.
Savvy skippers also tape anodized metal fittings (such as bow rail stanchion bases and cockpit ladder steps) to protect them from caustic teak cleaners. Frugal owners might opt for a powder cleanser, but oxalic acid will damage anodized surfaces. One hapless owner at my marina found this out the hard way when the cleanser dulled his metal fittings. He compounded the damage with a second pass because he thought the original treatment did not do a good job. It ruined the anodizing altogether.
That poor fellow has become my poster boy when I tell people to read the label twice on the container of any cleaner, paint or chemical before applying it to their boat.
This article originally appeared in the March 2025 issue of Passagemaker magazine.
