BTW yes that quick connect is for a side burner something that would have gone on the right hand side of the grill which I'll lay odds someone sawed off. I doubt you'll ever be able to make it "whole" again sadly. I see so many grills that people just cut the sides off of. Very sad. If you can find a 450 series of this grill you might be able to salvage the other side from that frame. Hopefully you got this for free or nearly so
Ospho rust treatment is what used to treat rust on steel salt water yachts. I would think it would work fine, then paint over it with high temp paint.
Don't know what to tell you especially since I don't know how deeply you're buried in the thing financially now. Sad a fine grill like that got butchered up
Really the hardest thing you have to find is that side table and burner that some idiot removed
I'm assuming you're thinking about something like POR15. I'd assume once the remaining material hardens it would be safe. Thinking about getting some myself