How to remove stains (tomato acid) from a Sabatier carbon steel knife?

Bar Keepers Friend is recommended.

http://sabatier-carbon-steel-knives.cookability.biz/care-use

make paste out of dawn dish soap and baking soda spread it on blade let sit then rinse with hot water baking soda can be abrasive so don't rub it on blade;wipe blades with wet cloth immedianely after using so they do not get ruined.

Caring for a carbon steel knife

Carbon steel blades are a popular choice for professional chefs; Japanese kitchen knives are traditionally made from carbon steel. Unlike stainless steel, carbon steel can rust if not properly looked after. This is because carbon steel knives do not have the protective chromium coating that comes with stainless steel knives.If food dries on a knife blade it can be difficult to get off, which can mean having to scrub the food off the blade – a task that’s not only dangerous (one wrong move and you could cut yourself) but which will compromise the sharpness of the blade. Acidic food can also corrode the metal, increasing the risk of rust. Again, wash, dry and store your blade as soon as you’ve finished using it.

The following tips will help to keep your carbon steel knife rust-free and in excellent condition for as long as possible:

• Wipe clean as you cook. For all knives it is important to wash immediately after For carbon steel knives, it is advisable to wipe the blade as you are cooking. This is especially important if you are using ingredients that are acidic, such as fruits, tomatoes and onions. The best way to do this is to take a damp tea towel, fold it neatly into a pad and place it on to your work surface. Then you can simply wipe both sides of the blade on the pad each time you finish cutting.
• Wash and dry before eating. Carbon steel is particularly susceptible to corrosion, so wash with soapy warm water before you sit down to eat your meal. Do not leave you knife covered with food while you eat.
• Hone before use. Keep your blade as sharp as possible by honing with a steel before you begin preparing.
• Remove rusts spots. Rust can develop on carbon steel blades. This should be removed as quickly as possible with water and a brush to prevent it from building up. Sustained rust can damage the metal; removing the rust quickly leaves the blade undamaged.
• Be prepared for ‘patina’ to develop. Patina is the discolouration that can occur when some metals react with oxygen. A common example of this is the greenish/blue hue that forms on copper that is left exposed to the elements. On carbon steel knives, this is usually in the form of a mottled grey appearance on the blade. Unlike rust, this effect is not harmful. While some chefs try to remove the patina for aesthetic reasons, others actively welcome the process, as this layer of oxidisation can work to protect the metal.