With rust the only cure is to remove it. Products like Por-15 are simple band-aid cover up jobs. They don't fix, cure, solve rust they cover it up.
My guess, is that moisture got behind those weld on drip rails and started to rust the metal behind the drip rail.
IMO, the best way would be to remove those drip rails because there WILL BE rust hiding underneath them. You likely will also find some pinholes and thin metal. Cut out the rotten metal and weld in new metal.
If that sounds like more work then you care to tackle, then I would use a spot blaster and blast that rust away.
http://www.handsontools.com/Unitech-...FY-1wAodrWsAMw