the clay we use is nonhardening clay. I used a layer between the baffle that the speaker is screwed to, and the metal door of the car. then i pushed excess up around the edges of the baffle, and covered all that in deadener to prevent possible running and to ensure it does stay soft. it helps to absorb vibrations from the speaker.
the clay most recommended is a white color and comes from dick blick's. i used some i got at hobby lobby. it stays soft up to 240 degrees.
this is great for an install, but goes hand in hand with sealing your doors well and making a solid baffle for the speaker to attach to.
