I think that the bondo you have is for more of fine detailing. It will work for those little areas where your fibreglass didn't cover or those areas where there is a small hole.
If you want to start off with what I did, I used something called a bondo fiberglass repair kit. It comes with resin, hardener, fiberglass mat, instructions, and a scraping tool. It helped me out a lot. If you look for that kit and can't find it, just ask the people at your friendly local hardware store for fiberglass resin and they should help you out.