Liquid sound deadener-how did you apply it to the engine compartment?

[lexicon]

I will be using liquid sound deadener to the entire car, but I also want to use it for the engine compartment (since you can not access the firewall completely from inside the vehicle)There is a cardboard covering on the firewall and inside of the hood.Did you remove this and put the sound deadener liquid on the bare metal, or apply the stuff over the cardboard material?Thanks

[idontknowwhatimdoing]

put it on the metal....adding mass to the panel dampens vibration and thereby reduces noise