Caring for Your Stainless Steel — FAQ

Our stainless steel cups are made from 304 (18/8) food-grade stainless steel — the same grade used across quality drinkware and food-contact products generally, containing 18% chromium and 8% nickel. This is a genuinely corrosion-resistant grade, not the cheaper 201 alternative sometimes substituted elsewhere to cut costs, so the guidance below is about ordinary care rather than compensating for a lower-grade material.

Not quite — it's rust-resistant, not rust-proof. Stainless steel contains chromium, which reacts with oxygen to form a very thin, invisible protective layer on the surface. That layer is what stops the iron underneath from rusting, and it's self-healing as long as it has ongoing contact with air. Rust appears when something breaks that layer down faster than it can repair itself, or physically blocks it from repairing at all.

A handful of very ordinary things can do this, even to good-quality stainless steel:

• Salt and chlorides. Dishwasher detergent, salty food residue, or salty water attack the protective layer in much the same way seawater corrodes metal — the single most common cause of surface rust spots on stainless steel drinkware.
• Acidic drinks or food left sitting. Citrus water, soda, wine, vinegar, or tomato-based sauces left in contact with the surface for extended periods can gradually wear down the protective layer in that spot.
• Trapped moisture. If a product is put away damp, or sealed with a lid before it's fully dry, the protective layer can't access the oxygen it needs to keep re-forming.
• Scratches, or contact with something else that's rusting. A deep scratch exposes fresh metal that hasn't built up its protective layer yet. Rust particles from something nearby — an old steel wool pad, a rusting rack — can also land on the surface and start corroding right there. This is often called contact or transfer rust, and looks like the product itself failing when it isn't.

Not necessarily — but it can be, so it's worth knowing the difference.

Genuine food-grade stainless steel (304, or "18/8," and 316) is highly corrosion-resistant and shouldn't rust under normal use. A lower, non-food-grade steel sometimes substituted to cut costs (grade 201) contains far less chromium and nickel, and can develop visible rust spots within days of contact with something as mild as lemon juice or coffee.

If you're seeing rust appear quickly, repeatedly, or spreading rather than staying as a single small spot, that's worth raising with us directly. A single small spot after a lemon sat in it overnight, or a run through the dishwasher, is a much more ordinary story and not a sign of a faulty product.

A small, isolated surface spot is generally not a food-safety concern in itself, and can usually be cleaned off (see below). If a spot keeps coming back after cleaning, spreads, or starts to pit or flake rather than sitting on the surface, stop using that item and get in touch with us.

• Baking soda paste: mix with a little water into a thick paste, apply, leave a few minutes, then rub gently with a soft cloth and rinse.
• Vinegar: soak a cloth in white vinegar, lay it over the spot for 10–15 minutes, then wipe and rinse thoroughly.
• A non-chlorine stainless steel cleaner (such as Bar Keepers Friend) for slightly more stubborn spots — follow the product's instructions and rinse well.

Avoid steel wool, wire brushes, or bleach — steel wool can leave metal fragments behind that go on to rust themselves, and bleach contains the same chlorides that cause rusting in the first place.

• Hand wash where possible — dishwasher detergent is one of the more concentrated sources of rust-causing salts.
• Dry it thoroughly after washing, rather than air-drying with water sitting in a seam or corner.
• Don't leave acidic or salty drinks/food sitting for long periods — a quick rinse after use goes a long way.
• Store it dry, with the lid off if it has one, so air can keep circulating.
• Keep it away from other rusting metal items during storage or in the sink, to avoid transfer rust.

Occasional dishwasher use generally won't cause a problem, but if you're noticing rust spots developing, switching to hand washing is the first thing worth trying — it removes the biggest single contributor (concentrated detergent salts) entirely.