IMHO, a wood hardener or epoxy wood be a better solution than filler*.
First, try to remove all the really loose and soft rot.
Then, you can get really thin penetrating hardener that soaks into the wood and stabilizes it. Often called wood petrifier or something similar.
Finally, there is viscous wood epoxy for sculpting and and building up the final shape.
*Maybe thats what the previous posters meant, but bondo is not a great option for exterior. Fiberglass bondo might work, but traditional bondo absorbs moisture and is meant to be underneath a water tight layer (car paint).