You can usually get rid of rust in a carbon steel or cast-iron pan by seasoning the pan again.
A pan like this, such as one by de Buyer or Petromax, needs a seasoned layer: a natural non-stick coating. Read how to season a pan. The process is the same for a cast-iron pan.
Properly seasoned, but still finding rust?
Acid, such as that in tomato purée, lemons or limes, damages the non-stick coating. Cleaning products damage it too, so do not use them to clean your pan. In either case, seasoning it again is usually enough to restore the coating.
Does seasoning it again not help?
Start afresh. Sand the pan with sandpaper (80 to 120 grit) until the bare metal is visible again, then season it again.
Think there's something wrong with the pan?
Send us photos of the problem. Describe how it happened and what you think caused it. Our specialist will then decide on the best next step. Read making a warranty claim to find out what else we need. Report it through the contact form.