I recently finished repairing large broken all of the way through bumper cover cracks.
After removing cover, I actually stapled the pieces back together in place (temporary) on both front and back side. Where possible I drill small holes in cover adjacent to the cracks and thread small zip ties to pull the cracks together. Went to home repair supply outlet and picked up some drywall seam fabric tape. This comes in different texture and I chose the heavy grade. I use Fasco Epoxies #88 2 part epoxy to affix parts permanently and fill cracks. Next step is to open up the cracks into a mini trough with a die grinder and burr bit. This allows the epoxy to actually fill and bond the pieces. Lay down a layer of the epoxy heavily. Cut strips of the drywall fabric and press them on on top of fresh layer of epoxy overlapping cracks by a wide margin. 5 minutes later lay a top layer of epoxy on top of dry wall strips overlapping entire fix area.
Next day is time for sanding out staples, excess fabric and epoxy. I do this on both front side and back side. Works great. On back bumper , it may be necessary to carve a notch in the styrofoam to ensure the cover lies flat when lt is bolted back in place. The repair on the inside may be bulbous. After final sanding, priming and painting with single stage urethane paint, it is flawless.