[Marvin/TN]

(Have you never seen rust under bondo?

Of course metal will rust under body filler. Plastic body filler is porous (spelling) and will absorb water if it gets wet which usually happens before any paint is applied usually due to wet sanding. Get a chip in the paint all the way to the filler and water will soak through to the metal and rust will start, it may take longer that way but it will happen. But in saying that I would use a quality plastic filler before lead in most cases. I refuse to use the word Bondo it was the original plastic filler and wasn't that great not like todays fillers. I have been using Plastic fillers since June 1966 as a professional.