ok for the rear end you need to grind it asll down or however u wanna go about it get it back down to bare metal and then start over...if you can weld up those hole from where the puller was used and then fiberglass over them. If you have rust on the inside yes you wqant to clean it up and then seal it up with some ruberized undercoating. also leightweight filler is regular filler just leightweight easier to apply and sands better and it will stick to fiberglass just fine as long as you sand the fiberglass with 80 grit or coarser i use 36 for fiberglass cause its quick and your just gonna cover it with filler anyway. as far as the body lines on the doors you can fill it with fiberglass but there is no guarantee that it will not crack cause that is simply alot of it to be putting on. but yes if you are going to fil the body lines be sure to get it up to the right height with the fiberglass then cover it lightly with plastic filler, cause if you use just filler it will just crack guaranteed. you will want to use something for a lieghtweight plastic filler for a coat over the kitty hair because the4 kitty hair doesnt sand out as smooth and nicley ans will take longer to try to make look nearly as nice. sand your filler with 180 or 220 then you are good to prime, just be sure to get out any 80 grit or coraser scratches before you prime because veen if you can sand them out of the primer and not see them anymore the primer will still shrink some and you probably will end up seing it eventually. I just repainted my car again this yr i will post up some pics of it in a day or 2 for u to see
