propane flame,putty knife and a bucket of water...carefully heat it up and scrape it off...no bondo dust or grinding...lifts right off... and yes I have pulled all sorts of stuff from behind bondo..coffee cans,paper,rags...thats what the bucket of water is for...in case something catches fire
purchased a parts car from a soldier shipping out really cheap because he said it was from the east coast and when he drove it it handled strange. some time went by after robbing the front sheetmetal and had sold the rearend. when the buyer started to lift the ass end we heard a loud snap and the rear 1/3 of the car breaks off! 2x4's inside the frame rails,pop rivets and sheet metal all covered in reasonable layer of bondo, only inches thick in places!
Um...he sold it to you! Sorry, but buyer beware! Best way I've found to remove it is to warm it with a tourch. a hand held propane unit will work fine. Warm just enough to scrape it with a puddy knife, not enough to hurt the metal. It's quick, easy, and you don't put it in the air.
Ive been working at a body shop/ restoration shop for about two years and ive already seen so many cars that guys thought were solid and then they slam their door hard one day and the bondo cracks out exposing rust or huge creases
My favorite bondo backers are used 5" self adhesive sanding discs. Saw a fly by night car lot body shop doing these "patches" on a car with rust holes in the doors.
Kid I knew told me years ago how he was in a shop where they were "restoring" a '60 Olds and they'd mix iron filings into the bondo so a magnet would stick to it. Needless to say even at 16 I knew better than to take a car there, I figured I could do just as terrible of hack work myself.
gotta love a bodyman in a can!!! heres an easy way to tell if theres over 1/16" of mud on anything......get o refrigerator magnet(you know the thin kind that dominos has their # on) if that wont stick its over 1/16" thick(yeah i keep one on my box at work) you wouldnt believe some of the stuff ive found mudded over,window screens,shoes,newspapers,card board,etc.