The patina look is very popular and you are correct, if you do not coat it, it will continue to rust and weather until it is functionally destroyed.
KBS MAXX Clear is the popular choice to clear coat a patina'd surface. KBS MAXX Clear provides incredible rust protection and KBS MAXX Clear is very durable and UV stable (does not yellow or chalk). KBS MAXX Clear also comes in several sheens (Gloss, Satin, Flat) meeting the matte preferences of many people looking to protect and keep a patina'd look. KBS MAXX Clear is also easy to apply by brush, roller, or sprayer giving plenty of application options for the fence.
KBS MAXX Clear Link --> https://www.kbs-coatings.com/kbs-maxx-clear
Please be sure to read and follow the application directions so you get the look and performance you expect. KBS MAXX Clear Application Directions --> https://www.kbs-coatings.com/kbs-maxx-clear-directions
Please let us know if that and reach back out if you have any additional questions.