How to Check For Bondo Body Filler in a Car Body

Before you buy that used car you may want to check to see if there was ever any body damage repairs done. Is it all metal behind the paint, or body filler too?
This process will detect larger filled areas but may not detect very small areas of filler.

Instructions

1

Squat down along the rear side of the car and look forward, keeping your eyes along the profile of the body lines, looking for any waves, dips, lumps or imperfections. They are clues that body filler has been used.

2

If you see any waves, dips or lumps, go to that area and use your knuckles to knock-knock-knock lightly on and around the suspicious area.
You want to hear a metal sound and not a dull thumping sound. Practice a bit ahead of time if you can; metal has a distinctive sound and body filler has a dull sound.
Ideally you should knock-knock along all sections of the car, particularly on the lower sides.

3

You also want to look very closely into the paint in suspicious areas to see if you can spot any cracks or splits in the paint, which could be body filler that has shrunk and left open splits.

4

If you are liberty to poke a little and you have your awl or pick with you, you could actually poke into an area and scratch out body filler if it is there.
If there is only metal behind the paint you will only scratch out the paint and layers of primer and hit metal. If scratching out body filler, it will be chunky and much thicker than just paint and primer.
A hot spot for bondo filler, especially on older cars, is just below the rear quarter panel, behind the rear wheel. This area tends to rust out, especially if the car came from an area where road salt is used.