Doesn’t Bondo eat polystyrene (but not urethane foams aka yellow foam aka balsa foam)? We are not allowed to use Bondo on campus so I am not familiar with it.
Another option is aquaresin – it will add strength if that is an issue, but could be expensive if you need to cover a large area. It can be dilluted and sparayed, and it is supposed to be non-toxic. You would not need to sand if you use this.
One step that is helpful, but many people neglect, is to poke holes all over the model before coating it with filler, which helps the filler to adhere.