I bought an old fiberglass jon boat to play round with and it has some cracks and holes in the fiberglass. The worse areas are where the tramson, side and bottom come together. To fix this I fabed some stainlass steel corners that I am going to install on the boat. the question I has is how to seal the stainless steel to the fiberglass, with resion or sealer like 5200

I would use the 5200. Even though it "permanent", it will still allow a little flex. Due to the aluminum & 'glass, there will always be some movement- however little. Resin would not allow any movement as it is rigid. Thus you would eventually end up with crack or crazing. Use the 5200.