Is the stain peeling or is it a clear coat that is peeling? If the stain and clear are both peeling then stripping off everything and starting over is the best option. It is possible to remove just the peeling stuff, using a stiff nylon brush or fine steel wool, and restain. This would be the same procedure as restaining a fiberglass door.
It really comes down to the actual condition of the finish. If you do a full strip then the door will need to be reprimed with an appropriate color primer before restaining.