It turns out the leaky sealant in the drip rail was also the only thing holding the roof skin to the vehicle.
With the drip rails clear, lifted the the roof to treat the otherwise inaccessible rust.
Plenty of rusty flakes
A section cleaned up with a wire wheel. Thankfully the structural areas were still sound.
Here is the roof and drip channel ready to be made into a POR-15 sandwich
With everything done, rusty roof areas were:
degreased with simple green
treated with POR-15 metal prep or Ospho.
left to dry
drip channel painted with POR-15
roof skin refitted
painted top of roof flange with POR-15
sealed with a good quality seam sealer.