F - Filler
Filler - This useful substance is used to 'fill' the gaps in a car's structure where metal has corroded, or after accident damage. If, on close inspection, a car's body panels appear wavy and uneven, the chances are filler has been used.
Tap the panel with your knuckles. If you hear a dull thud rather than a metallic ring then the panel's full of filler. If it's just a small area this shouldn't necessarily put you off buying the car, but if it's extensive, walk away.