I just clean it up (remove the surface rust) with a dremel or wire wheel on a drill and paint it with anything handy. The biggest part is covering it. About any rattle can primer makes me feel better.
What do you think about using POR 15 for this application? The big openings that I can get at with the dremel won't be an issue, but access to the small openings, and especially the various screw holes, will be tough. I was thinking that POR 15 on a Q-Tip swabbed into the screw holes might be an effective way to get access and coverage both through the hole and on the edges immediately behind where there is no access.
I'd say POR would be superb, or Zero Rust or Rust Bullet or any of those. Of the fancy 'one and done' rust products I've only used POR and there is a significant bit of prep. That's my only hesitation. If you don't mind sanding, cleaning, degreasing and then metal prepping then POR is amazing stuff in my limited experience.
I use fluid film on on my vehicles, seems to work very very well if you start undercoating before the rust start. On my vehicles that have a little rust to begin with its done a decent job of keeping it at bay. Any paint style undercoating will only cover the rust, but lets it continue underneath where you cant see. The exception is something like POR-15 but thats got a pretty long prep process.
Same issue as Kelp on a 2014 Sprinter. Found it when the paint bubbled. Electrical side much worse that water inlet side. Obviously SMB isn't doing even basic corrosion prevention which would prevent or at least slow this down. My advice is new buyers should insist on something and if your beyond that stage check it out.