this is how i repair plastic bumpers in work.
in this case it was a repair to a ford focus rear bumper that someone had repaired and it failed.
they used a piece of metal and rivets and filled the outside and fibre glassed the inside. this usually doesnt work.
i use this stuff.
you buy it as a kit for about £150. or just the tubes of replast for about £17.50.
1. firstly remove all old filler and fiber glass. then 80 grit inside and outside. and trim the edges with a stanley knife.
im not sure how many pics i can put in one post so will do this in stages