Roofing tar would probably be a more cost-effective alternative to what I used. Permatex makes both asphalt based undercoating and rubberized undercoating under different part numbers as shown below. I used the 80072 version. Both are messy to use, so wear disposable nitrile gloves when spraying.Roofing tar, then? Not sure I’ve seen non rubberized, tar-like undercoating.