[SlowMopar]

Ok. What you use would have to depend on what the peice is made out of. Bondo is not a good idea for a bumper cover. There are several makes of putties that can be used for plastic or eurothane (sp) bumpers. Should have it at any local car parts place. Plastic bumper repair can be tricky. Some will require the cover to be melted around the area to be repaired. I't been a long time since I worked in the body shop but I'll swing by and get the name of the stuff I used to use.
For ground effects it once again depends. Fiberglass peices would best be repaired by fiberglass matt and epoxy. The a LITTLE bit of bondo to make it smooth. I'm talking a very little bit.
For the eurothane (once again sp) see above. Hope this helps a bit and I'll find the name of that product for you. Any time anyone has a paint or body question feel free to PM me. I was top of my class in Autobody and worked in a body shop as a painter/body man for a long time.