GQ Patrol rust issue

I have a '91 GQ Patrol. Until about 2 years ago, it was always parked undercover, and no rust was apparent.

Now that it is parked in the open, some corrosion has emerged in a seam below the windscreen, where the pillar meets the firewall.

Any suggestions how to tackle this please? Should I clean up what I can get to, and try to get fish-oil into the area? Would a ERPS type system help, or is it too late for that?

Other than this, she is still in really good shape. I would love to get another 5 years out of her at least.

Cheers,
Rob