I'm no expert, but if it were my bumper, i'd be stripping back a section, and using something like plasti-bond to fix the crack. Then sand it all down, and re spray.
Iv'e used plasti-bond on heaps of stuff before (including wood, metal, plastic and concrete), and I love the stuff!
if you use epoxy, make sure it's not quick dry, as quick dry is brittle. If it's quick, dry mix it so you use less harder. This should make it less brittle.