How to Clean a Fiberglass Bathtub | Protect the Gel Coat

For the full breakdown, see our best Cleaner For Fiberglass Bathtub guide.

The wrong approach — steel wool, scouring powder, or bleach — can dull the finish fast. The right one takes about ten minutes and leaves the tub looking new without damaging the material. Here is the step-by-step method that works on routine dirt, soap scum, and light stains.

The Safe Daily Cleaner: Dish Soap and Warm Water

Warm water and liquid dishwashing or laundry detergent is the most consistently recommended DIY method. It is effective for weekly maintenance and gentle enough for any fiberglass surface.

2. Wet the entire tub with warm water first — this helps the solution spread evenly.
3. Apply the soap mixture with a soft sponge or microfiber cloth using circular motions. Use a soft toothbrush only for tight crevices, and keep pressure gentle.
4. Rinse thoroughly. You may need to rinse 3 to 5 times to remove all soap residue — leftover film attracts dirt.
5. Towel-dry the surface. Drying prevents mildew, mold, and water spots.

Before you start, open the bathroom door and window and run the exhaust fan. Good ventilation speeds drying and clears any cleaner vapors.

Common mistakes to avoid: Never use steel wool, scouring pads, abrasive powders, or harsh scrub brushes. These scratch the gel coat and create permanent dull patches. Also, do not use bleach: it can damage fiberglass over time, especially if left to soak.

Removing Buildup and Stubborn Stains

For soap scum, hard water marks, or light stains that don’t respond to the dish-soap method, three pantry ingredients handle most situations. Each targets a different type of mess.

• Baking soda paste. Mix 3 parts baking soda to 1 part water (or 1 cup baking soda plus 1 tablespoon water for a thicker paste). Apply it to the stained area and let it sit for about 30 minutes. Scrub with a nylon brush or soft sponge, then rinse thoroughly. The mild abrasion lifts scale without scratching the gel coat.
• White vinegar and dish soap. Spray the area, let it sit 10 to 30 minutes (soiler surfaces need the full time), then scrub with a non-scratch sponge and rinse.
• Hydrogen peroxide. For dark or set-in stains, soak a cloth or paper towel in hydrogen peroxide, place it directly over the stain, and leave it overnight. Rinse with cool or warm water in the morning. This is effective for discoloration that the other methods miss.

One caution with vinegar: Avoid letting any acidic cleaner — vinegar or commercial products — sit on one spot for hours. The gel coat can “burn” or etch if exposed too long. The 30-minute soak is safe; overnight is not.

Maintenance That Keeps the Finish From Dulling

Fiberglass is durable, but the gel coat is a thin layer. Three routine habits make it last years longer.

• Rinse and dry after every bath or shower. Standing water leaves mineral spots, and leftover soap film turns into stubborn scum. A quick towel wipe after use prevents both.
• Use a daily spray cleaner between deep cleans. A mild, non-abrasive bathroom spray (or the dish-soap mixture in a spray bottle) can be used daily. Spray the walls after each use and let it run down with a quick rinse — no scrubbing needed.
• Never let cleaners pool. If you use a wax-based product or any finish restorer, follow the directions closely and do not let it sit in one area for more than a minute.

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