undercoating inside the doors and sound deadener

burdar said

There is already factory spray-on undercoat inside the doors. I'm using the foil backed butyl(Fatmat/Noico/Kilmat) type sound deadener on the floor and was going to use it inside the doors. Now I'm questioning whether it's needed in the doors since there is already undercoating there.

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Most older cars I've done usually have old dried out undercoating there but the factory undercoat almost always doesn't cover the very bottom of the door. That's why doors always rust at the bottom, plus the drain holes get plugged. Clean the doors out real good and see if there's undercoat at the bottom of the doors!