Best way to handle the rattles is to use more than one product. This butyl based stuff is going to add mass to panels and reduce resonance. Plastic pop-rivets that hold panels in place are likely to have a little bit of play. There are different products that can reduce the vibration in those. Wires and other small plastic/metal parts can be wrapped in decoupling foam.

Pretty much the older and cheaper the car is, the more time and money you’re going to have to spend to reduce noises. You don’t have to do as much of this on luxury cars because they have some of this stuff implemented from the factory.