If you can take it off with your fingers, it might just be slightly "glued" to your tin' surface.
Please pay attention to the following products because the mix can be really aggressive for your eyes and skin. Equip yourself with protection gloves and glasses before any other step and for the whole operation.
I would suggest to use white vinegar and sodium bicarbonate (AKA baking soda), which are both pretty easy to find (i.e. in your local supermarket). As a dosing ratio, I recommend 15-20g (1 tbsp) of sodium bicarbonate per liter (33 oz) of vinegar.
- Get a bigger container in which your baking tin can fit, leaving space around and over it. Put your baking tin in it. and
- Add the white vinegar until the surface which you want to clean is fully covered.
- According to the given ratio, add your sodium bicarbonate. The mixture might react because of the carbon dioxyde being produced. Thus, make sure not to put the soluton in a bottle of closed container. If a foam appears, no worries : it is normal and non-toxic, but irritating.
- Let the mix do its job for 1 to 2 hours and you should then be able to scrub your surface with a sponge withouth scratching the metal.
- Make sure to rince very well with clear water and normal soap before any cooking.
- Optional : if you need the mix to be more efficient, add a bit of lemon juice into it.
Note that this operation works really well with greasy and stained surfaces and clogged pipes. It is 100% natural.