This was what I was originally considering but the fiberglass won't crack or pull apart from the metal over time? It's going to be covered by vinyl but I'd hate to have a divit or depression form because either the bondo or fiberglass separated from the metal.
If the metal is clean, I would think the fiberglass would hold. You would have to wash it with paint thinner or acetone to insure there wasn't any oil, not even from your skin on there. The roof really doesn't flex much, unless you are putting out ungodly amounts of horsepower and twisting the entire car.
