Hi Julian,

I suggest taking the approach recommended by Mel and Dave, but with a caution about Bondo. First find out the source of the water. I suggest taking off the door and removing the lower hinge to see how far up the rot extends. If it starts at the hinge and the wood is solid above the hinge, then water likely entered from the hinge screw holes. But try to determine exactly where the rot starts. Assuming the wood is sound above the hinge, I suggest you take one of two approaches, depending on how deep the rot extends. If the rot extends through most of the wood, both front to back and from what was originally the outer face to the styrofoam , then I suggest you remove all of the wood below the point where the rot starts. At the bottom of the sound wood, cut a scarf of about 4" length (top to bottom ). Then cut a new piece of wood to fit in the cavity with a matching scarf to mate with the remaining wood. Roughen the front and back fiberglass skin where the replacement wood will be inserted with sandpaper, then tack off the dust. Use a good quality epoxy, such as West System, to epoxy the replacement piece to the front and back fiberglass skins and for the scarf joint. I recommend not using five-minute epoxy from the hardware store or a big-box store since it has relatively little strength compared to quality epoxy. If the replacement wood is not a tight fit to the fiberglass skins, then use a mixture of epoxy resin and colloidal silica to make a paste that will fill any gaps and will not run out of the joint before the epoxy sets. I suggest not using pressure-treated wood since the epoxy will not bond as well to PT wood as untreated wood. Cut the rabbet for the hinge, then prime and paint. Caulk each hinge screw hole before installing the hinge.

The second approach is if the rot does not extend all the way from front to back and from the outer face to the styrofoam. In this case cut out the rot, then cut a piece of wood to replace the rotted stuff. The fit doesn't need to be perfect. Glue in the filler piece using a non-sagging stiff paste made of high quality epoxy resin with colloidal silica filler. Coat all surfaces with this paste than install the filler piece and screw it into place so the epoxy can set. The excess epoxy mix will squeeze out, filling all the gaps. Leave the squeezed out mix in place. After the epoxy sets, remove the screws and remove the excess filler with a rasp or file. If there are still some gaps, you can use Bondo to fill them, but since you have the epoxy and silica, I would just use that material. Bondo has very little strength. I suggest you not consider filling a large area with Bondo, especially around the hinge area where strength is required.

This repair should take only a few hours of actual work time, not counting time required for the epoxy to cure. While replacing the door with a new slab sound much easier, you may need to make adjustments for the lockset and hinges, which could take some time. With a repaired door, you know if will fit the frame as well as it ever did.

Good luck.