If he were painting the whole bumper, yea, ide say use a
flex agent, but it sound like its just a 2 inch spot...no need for the agent IMO...
I have sprayed a couple bumpers without, just because the clear was flexable in itself and the manufacture recommended not to use it...I was mistaken, it doesn't expire in a couple days, but doesnt last forever...people have mixed feelings on it though, some think its a joke, some praise on it...