Is there a rust remover that is safe to use on a rat cage?
I suggest you change to a glass aquarium like the size of a 10 gallon one or larger. You can also get a metal mesh top on the cage that will secure in place. There will be no deterioration of metal this way and it is much easier to keep clean. Change the bedding often and you won’t have urine smell. Pets are only good as they are provided for. A rusty cage is not healthy for your pet and trying to use chemicals to clean it will leave a residue and also not healthy.