I would try sending the rust off if you can. You may end up having to paint parts of it, but the side with the company name looks perfect and could be left alone. You could spray paint the rusty sides with Rustoleum once you get the rust off, but again, the focal point will be the company name side which can be left original. Good luck, Hgj! It will look great on your porch when it is finished. Leave the front alone, but you could seal it with a clear coat (Rustoleum has a clear top coat) to keep it looking good if you want.
Try Tarnex after light sanding rusty areas. If sanded areas come up brightly polished you can coat the area with toilet cleaner and let it set for a few hours and then buff off. I personally like Krylon clear coats they come mstte, satin and high gloss, and hold up outside.
I have not personally use this product personally , but have had exceptional experience and performance with other Rustoleum colors and sheens. This may be an option for you —consider looking over user reviews and QA and see if that might meet your need.
Rustoleum make spray paint he looks like galvanized metal and it also is a rust preventative.
I have the same milk box found in an antique store I use it to put my garden tools it sits on my front porch