My old 89' Legacy GT wagons bumper is looking pretty sad. PO must have pranged it a little right on the corner and rippled the plastic on the bumper just below the corner light. Seems like they've put filler on it and sprayed it, but whilst swapping the fender some of it has cracked and come off.

 

Seems the filler that's been used is just regular body filler and doesn't like a small amount of flex. Is there any proper filler made for being applied to flexible plastic?

You should be using a polypropylene plastic welding rod that is melted into the holes/gouges on the bumper.

 

The factory manual gives a little bit of instruction/guidance. Starting on page 79 of 206 for the Liberty manual you can see what it says.

 

http://www.main.experiencetherave.com/subaru_manual_scans/1992_Liberty_FSM_-_FULL/1992_Subaru_Liberty_Service_Manual_Section_5.zip