body sealant??? stuff for truck and floor pans

I was just curious what everyone does for the sealant on the trunk pans where the peices are fitted together. Some of mine has peeled up and fell apart. I plan on having the car painted soon so i figures i should reseal the bad areas.

What exactly is the stuff?? Where can i get it??

And does everyone peel up all the old sealant and replace it or just the loose and missing parts??

Thanks for any info