at first i tried to put a plate on one side, then fill from the other. That didn't work. The way I ended up doing it was use the caulk gun from one side, and just push as much in as I could. I might have used electrical tape on the other side, but right after i got one side completely filled, I turned it over and filled the other side. Then I used my finger to smooth it out, flat with the edge of the mount. That way all the air was out, and it filled up the whole mount.
It was really messy, and I used thinner to remove the tar from my hands. It got everywhere.
It was really messy, and I used thinner to remove the tar from my hands. It got everywhere.