I have read all the comments carefully, but as I said at the beginning , here I only find products like Perma Blue. I have searched and read many google sites on how to make a proper treatment, as usabaker comments, but I do not feel able to do something like that. For now I will start cleaning the entire surface of the receiver, while analyzing what kind of protection against the rust applied. I have possibility of using automotive paint, although I prefer the spray can.
Many thanks for your helps!!