Using Bondo for Wood Repair

I decided to cut out the rotten wood and replace it. It's a better overall fix, and it's not really any more work than trying to work bondo.

For the sake of the discussion though, when I was exploring ways to form for the bondo, I used some scrap 1/8 plywood and wiped a smear of petroleum jelly across it. I let it sit long enough for it to absorb into the wood and then I wiped the excess. I applied some bondo across the board. After it cured, the part that was over the Vaseline part peeled off smoothly. The rest of it bonded tightly to the plywood.
No Bondo, George Jets was bang on, but repairing/proper scabbing/sistering is the right way to go unless something prevents it. If it would require destruction of plaster/masonry to repair, then what George posted is the best way forward.