It is possible to find rust on the inside of a sheet steel (for instance deBuyer) or cast iron pan (for instance Petromax). These types of pans need to be seasoned before use to create a natural non-stick coating. You can read more about seasoning a pan here, the same applies to cast-iron pans.
Properly seasoned, but still finding rust?
Acids help shape rust by 'attacking' the non-stick coating. Think, for instance, of cooking with tomato puree and acids from lemons or limes. Often all you need to do is season the pan again to restore the non-stick coating.
Sometimes pans are also cleaned with detergent, whoops! That is not a good idea. Here the same applies as mentioned before: sometimes all you need to do is season the pan again to restore the layer.
Seasoning the pan again is not helping, what now?
No problem. Then it is best to simply start again. Sand down the pan with some sandpaper (grain 80 - 120) until it is 'blank' again and you can season it once more.
Still feel something is not right? Send us pictures of the problem and properly describe what happened and what you think is causing the problem. Our specialist will take a closer look and decide what the next step is.