Illustration for Boat Mildew Spots and Waterline Stains

This page is about getting stains off. For what a pro charges to compound and wax, see per-foot boat detailing prices. For what an owner should expect to pay, see why boat compound and wax quotes run high.

Two questions owners keep asking

In a popular boating thread, an owner asked why black spots keep appearing all over a covered boat. Could anything remove them without a buffer? Another owner asked about yellowing on a newly bought skiff. Degreaser and wash-and-wax soap didn’t touch it. Hydrochloric acid burned off the black deck print, and the yellow stayed.

Plenty of owners face the same thing. Maryland had 168,202 registered recreational vessels in 2024, according to the Coast Guard.

Which stain do you have?

Black or gray specks, often in shaded spots under a cover. That’s usually mildew. A boat restorer in the thread said the spots were mildew. A thick, chalky oxidation layer lets them grow back faster.

A yellow-brown band at the waterline. That’s usually stain from the water itself. A California water-quality guide says tannins from slowly decaying plants give water a tea color. It adds that iron causes red and brown colors. Lakes ringed by pine forests or wetlands tend to be the brownest.

Even yellowing or chalk everywhere, not just a band. That may be the gelcoat itself aging. One boat builder’s manual says gelcoat should last about 3 years in typical conditions without significant yellowing or chalking. Its fix for discoloration or oxidation is polishing and buffing. Severe cases may need wet sanding first. No stain remover fixes that.

What to use on each

Stain Try first Then Skip
Black mildew specks Boat soap and a soft brush Mildew stain remover for fiberglass (e.g., Star brite Ultimate Mildew Stain Remover) Household bleach
Yellow-brown waterline Boat soap, to see what’s dirt An oxalic acid hull cleaner (e.g., Star brite Instant Hull Cleaner) Hydrochloric or muriatic acid first
Even yellow or chalk all over Wash, then a test spot of polish Polish, buff and wax, or a pro Repeated acid passes

Mildew. One mildew remover maker says to spray or wipe it on and let it work until the stain disappears. Then rinse well with fresh water. One coat usually does it. Stubborn spots may need a second coat and light scrubbing. The same label says not to use it on wood or painted surfaces. Avoid prolonged contact with metal, too. Rinse off the trailer and any fittings it touches.

Waterline. One hull cleaner maker says its oxalic acid formula removes yellowing, scum lines and leaf stains. It works on fiberglass and painted hulls. Wear rubber gloves, leave it on for 2 minutes, then rinse with fresh water. Severe stains may need a second coat and light scrubbing. It works best at 10°C (50°F) or warmer.

Why not jump to strong acid? Another maker’s hull cleaner ships as a corrosive liquid containing hydrochloric and phosphoric acids. The skiff owner above said hydrochloric acid burned off the deck print. One boat builder tells owners not to use household cleaners, chlorine or acids on the boat at all. Our guide to using acid brighteners on stainless truck bodies the right way shows how strong acids etch metal, too.

After either cleaner, put wax back on. Maryland’s clean boating tip sheet says a good coat of wax keeps surface dirt from becoming ingrained.

Keep runoff out of the water

Do this job on land, not at the slip. Maryland’s Department of Natural Resources says to use soaps that are phosphate-free, biodegradable and non-toxic. Use them sparingly. It says to avoid detergents with ammonia, sodium hypochlorite (bleach) or lye. Never dump cleaning products down the thru-hull drain; dispose of them on shore. The EPA warns never to mix chlorine bleach with ammonia cleaners. Toxic fumes could result.

Detailers doing this for a fee should read our wash-water rules for mobile detailers. This isn’t legal advice. Check your marina and your state.

Keep mildew from coming back

The EPA says the key to mold control is moisture control. One boat builder says to cover the boat with canvas. Plastic and other non-porous materials trap moisture against the surface. Put the cover on a dry boat, and open lockers so air can move. Our pre-shrink-wrap clean covers venting a winter cover. Seats with mildew are a separate job; see chalky pontoon panels and stained boat vinyl.

How we check every number: methodology.

Sources

Based on 9 sources · How we price

Checked 2026-10-09.