Mould Stains in the Kitchen: Sink Caulk, Grout, Fridge and Dishwasher Seals

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Where mould stains show up in a kitchen

Health Canada lists kitchens among the common problem areas for moisture and tells you to check window sills and around sinks and pipes. Mould needs water that stays put, so it shows up on the caulk, grout and rubber seals that never fully dry.

The appliance seals surprise people most. LG says the condensation from opening the fridge (cold air meeting warm kitchen air) can lead to mould on a gasket that isn't dried. Whirlpool says mould can grow in the crevices of a dishwasher's gasket and cause a musty odour.

First: is it a stain or a moisture problem?

A few black dots on the caulk behind the faucet are routine upkeep. Mould spreading on drywall, inside a cabinet or around a window frame is a moisture problem, and no cleaner fixes that. Health Canada sorts mould by size:

On its Mould page, small areas (1 to 3 patches, each under 1 m², or 10 sq ft) and medium ones (more than 3 patches, or 1 to 3 m²) are yours to clean with water and dish soap, wearing goggles, an N95 mask and gloves.

Over 3 m², or if it keeps coming back after cleaning, consider a professional. Health Canada's one-page mould infographic is stricter: it says to hire a professional once the area is larger than 1 m². When in doubt, follow the stricter rule. Stains on kitchen caulk and seals are far smaller. Renting? Health Canada says to talk to your landlord.

How to remove mould stains from kitchen sink caulk, step by step

You need: dish soap, a soft sponge or nylon brush, a towel, gloves, eye protection and a gel stain remover. Time: about 4 to 6 hours, most of it waiting (our estimate).

1. Clear and wash (5 minutes). Move dishes, food, the drying rack and sponges away. Wash the caulk with warm, soapy water and rinse. For small patches, Health Canada says soap and water is the first step.
2. Dry it completely (about 30 minutes). Towel it, then let it air-dry. BLITZ says the gel sticks to a dry surface.
3. Apply a line of gel (2 minutes). Put on gloves and eye protection, open a window, shake the bottle and run a thin line of BLITZ Mold Stain Remover right on the stain.
4. Let it sit 3 to 5 hours. Don't run water in the sink, and keep the window open or the range hood on.
5. Wipe, rinse and dry (5 minutes). Wipe the gel off with a damp cloth, rinse with clean water, then rinse the sink basin before it touches dishes again.

Old, deep stains may need a second round. If the caulk is still black after that, the mould is inside the silicone, or the caulk is peeling and letting water behind it: remove it and recaulk on a clean, fully dry joint. Never run new silicone over stained caulk.

We don't have kitchen photos from customers yet. Here's the gel on a customer's bathtub caulk, the same kind of silicone joint as around a sink:

Grout, fridge gasket, dishwasher seal, window sill: surface by surface

Always start with soap and water. Use the gel only on the dark stain that stays.

On the fridge, do it when the fridge is nearly empty. The door closes on the gasket while the gel sits, so wipe the cabinet frame it touched when you remove the gel (our suggestion). Dishwasher smell too? Our dishwasher smell guide covers the filter and seal. Black stains on a washer seal are in our front-load washer gasket guide.

What not to do (and what never to mix)

BLITZ Mold Stain Remover is a bleach gel: its ingredients are sodium hypochlorite and sodium hydroxide. Health Canada warns that bleach reacts with any cleaner containing ammonia or an acid, including vinegar, such as glass cleaners and toilet bowl cleaners, and releases toxic gases that cause burning eyes and light-headedness and can be deadly.

• Never mix or layer cleaners. Don't use the gel at the same time as another cleaner, and rinse well between products.
• Don't scrub silicone with steel wool or an abrasive pad: it scratches and tears.

Mould under the kitchen sink: find the leak first

Mould under the sink almost always means water. Health Canada says to repair leaks as soon as you notice them and to make sure sinks are sealed tight so water doesn't leak into the walls. Check, with a dry paper towel:

• the P-trap joints and slip nuts under the drain;
• the hot and cold supply lines and their shut-off valves;
• the dishwasher drain hose and the garburator connection;
• the faucet base and sink rim, where water seeps through failed caulk.

Tighten or replace what drips, then let the cabinet dry with the doors open. If the drip is inside the wall, the cabinet floor is soft, or the mould is spreading on drywall, call a plumber.

Habits that keep kitchen mould stains from coming back

• Run the range hood when you cook and let it run a few minutes after. Health Canada says kitchen fans should vent outdoors.
• Keep humidity between 30% and 50%, Health Canada's range. A hygrometer from a hardware store tells you where you are.
• Wipe the sink edge and backsplash dry at night, and don't leave a wet sponge or the drying rack against the caulk.
• Wipe the fridge gasket when you clean the shelves, then dry it.
• Leave the dishwasher door ajar after a cycle so the seal dries.
• Fix drips the day you see them, and recaulk the sink when the bead starts peeling.

Why BLITZ Mold Stain Remover for kitchen caulk and seals

• A gel, not a spray. It stays on the vertical bead behind the sink and in gasket folds for 3 to 5 hours instead of running into the drain. More in mould gel vs bleach spray.
• Made for these surfaces: silicone caulk around sinks, tile and grout in kitchens, and fridge and dishwasher seals, per the product page.
• Rated 4.7 out of 5 (13 reviews) on goblitzclean.com. One wrote that it "brightened up the silicone caulking around my kitchen sink."
• 236 mL for $24.95 from a Québec company in Laval, with service in French and English. Free shipping in Canada from $40 ($7.95 below), 10% back in store credit, a 30-day money-back guarantee, most orders in 2 to 3 business days. Also on Amazon.ca.

Frequently asked questions

Why does the caulk around my kitchen sink keep turning black?

Water sits on it every day, and the mould may be inside the silicone or behind a bead that has lifted. Clean and dry it, use a gel on the stain, and if it comes back within weeks, remove the caulk and recaulk on a dry joint.

Can I use bleach on mould in the kitchen?

Yes, on the dark stain that stays on caulk, grout or a rubber seal after you wash it with soap and water. A bleach gel like BLITZ Mold Stain Remover clings to the stain for 3 to 5 hours instead of dripping into the sink. Never mix bleach with vinegar, ammonia or another cleaner.

How do I get mould stains off a fridge door seal?

Wipe it with a warm, damp cloth and dry it, as LG recommends. For stains that stay in the folds, apply a gel stain remover to the dry rubber for 3 to 5 hours and wipe it off with a damp cloth.

Is mould under the kitchen sink dangerous?

Health Canada links mould and damp homes to eye, nose and throat irritation, coughing and wheezing, and says to clean it up right away and fix the moisture. Under a sink, that means finding the leak first. Call a professional if it keeps coming back or covers a large area.

Is it safe to use a mould stain gel near food?

Yes, with care: move food and dishes away, open a window, wear gloves and eye protection, and rinse the surface and the sink with clean water before you cook.

Read next: Black Shower Caulking and Grout · Why a Gel Beats a Bleach Spray · Mould on a Front-Load Washer Gasket

Sources: Health Canada (Mould page, mould infographic and the bleach mixing advisory), LG and Whirlpool support pages, and the BLITZ Mold Stain Remover product page, checked October 20, 2026.