The issue with doors and fiberglass is that you are generally going to make the problem come back unless you replace the rusty metal with new. Doors are made to have water draining down the inside and out the bottom, and since they rusted in AZ I would suspect that the drain holes plugged up at some point. The fiberglass will certainly not help this and will probably trap more moisture against the remaining good metal.
You ought to be able to find some good doors in AZ, or you could try to repair what you have.
