I had never heard of backer rod.  When we had to do a lot of caulking, I found out about it.  HUGE home run.  I used to just keep filling cracks with caulk til they "filled up".  Now I ram this stuff in first, and then caulk over it.  Saves super big on caulk costs.  Easy to use.  Just pull off what you need and ram it into the crack with a scraper.  I got three sizes - kind of a small, medium, and large for different sized cracks.