New sealant will not stick to old sealant, so you will to scrape everything down to the glass.
GE silicone 1 is fine but RTV 100 series is much stronger and (in my opinion) a bit easier to work with. RTV can be purchased from grainger or other industrial supply type stores.
RTV 103 (Black)
RTV 108 (Clear)
We use this stuff at work to bond aircraft transparencies (canopies) which fly at mock 3... its pretty strong stuff.
Its also the same stuff they use to bond your windshield to your car.