The technique used to fill large gaps is called caulking.
Have you ever been in a position where you didn't know how to fill a large gap?
The challenge was still the same, even though it may have been the case with wood, concrete, or on the interior.
It did not seem correct to use a gap large enough to fit three tubes of caulk.
Taking the most appropriate thickness of backer rod and stuffing it into the crack that you are attempting to fill leaves a very small portion of the gap unfilled.
It is not meant to be a solution for gaps that were ultra-wide and would require finishing with a material other than caulk.
I've seen backer rod used for a lot of different scenarios, from gaps in trim to cement to window and door applications.
If backer rod isn't an option, you may have to use the multiple layer of caulk route.
One could fill the gap with layers of caulk, letting the caulk dry or set each time before covering it again with another layer.
It is important to use a backer rod that is a tad wider than the crack you are using it to fill.
If you want to trim the rod to the length you need, you can use a utility knife or razor.
It is important to not fall into the trap of thinking that backer rod is ok to leave a gap without filling it.
By using caulk, you will be able to form a seal around the backer rod and leave a smooth finish over the gap.
Having a backer rod at your disposal will make the task go away as fast as you can fill the gap, even though it can be a little bit of a head scratcher at first.