I would defiantly go with
epoxy, you will not need that much, likely a one gallon can. If you have some cracks they can be filled with thickened
epoxy and it will be stronger than the
wood. You will also be saturating the
wood with clear epoxy before you glass it and that will seal the wood to moisture.
Even if it gets wet Douglas Fir is pretty
rot resistant so I would think you are looking at at least 15-25 years out of this
rudder.
Good luck.