Hi. Personally if I were to do protect that surface and without access to 2k paints etc I would

Wire wheel the surface rust with a wire wheel in a drill or on a grinder
Kurust the whole panel
Prime the panel with red oxide primer
Apply seam sealer such as tiger seal or brushable seam sealer to the welds
Apply a further coat or red lead
Paint the patch with coach enamel in a colour of your choice (can get it from car paint stores)
Then underseal it

Or you could alternatively use stone chip and paint over the stonechip dependant on how far you want to go with it.