Sometimes wood repairs call for something faster and harder than wood filler. Like, when my shutters ended up getting Dry Rot Damage. The damaged area was so large and thick, that Bondo made the most sense for this repair.
Bondo makes an All Purpose Putty, that works on wood and a wood filler. I went with the putty this time because it dries super fast and a little harder than wood filler. And, because I am painting these shutters. If I needed to stain a large wood patch job. I would have used all wood filler or at least a good 1/4" surface of wood filler with Bondo underneath.
I have read online some people saying that Bondo cracks and shrinks. Maybe that's true of a different Bondo product or in areas with extreme cold. Our house painter used Bondo to patch a porch column of ours a few years ago and it has held up perfectly. But, I'm in Houston. So, we generally only have a handful of days around the freezing mark a year. So, with that said. Let me tell you what I did and recommend. More details about this tutorial can be found on my site.