can i use this to patch and repair a outside fiberglass mobile door?

no not designed for that application . You can pick up two part epoxy and some fiberglass cloth and patch the door. First scratch up the outside area before applying the epoxy. The folly the direction on the can for mixing.

Don't put much stress on the joint. This epoxy is not as strong as the original piece. If you do try it, sand the mating surfaces to give the Weld something to grip into. Then clamp in place to squeeze out as much of the epoxy as you can.