i had the same with a pug 103 I restored, it would clean up good then rust over again within a day, like others have said as long as the tank is not leaking and you have got rid of all the large flakes of rust (if not fill with the required cleaning product and add a bag of 1cm metal nuts and give it a good shake) just fill it with mixed gas and oil to the top as soon as its cleaned and fit a filter after the gas tap, the fuel may be orange for a couple of tanks of gas but it will eventually settle down as the oil lays a coating. just change the filter every so often. I have attached a pic of the type that would be best as it will trap all the small particles.
ensure you but the correct sized ones as thy come in lots of different sizes and pipe sizes. On ebay or similar you should be able to get a pack of 5 for a few dollars.