Complex sentence: The Bondo brand of filler or putty is composed of a polyester resin that, when mixed with a hardener (an organic peroxide) or catalyst, turns into a putty which then sets and hardens.
Split into simpler sentences: The Bondo brand of filler or putty is composed of a polyester resin. When the resin is mixed with a hardener (an organic peroxide) or catalyst, it turns into a putty, which then sets and hardens.