Body filler: when to use it and when not

‘They just filled it.’ You have probably heard that said about a repair, usually as a criticism. It is true that filler was misused for years to pack out dents nobody bothered to straighten. But in a well-done repair it is a necessary material. What matters is how much is used and what for.

What it is and what it does

Body filler is a two-part paste that hardens once mixed. It is applied in thin layers over metal that has already been straightened, to correct the small irregularities left after the panel work. Once cured it is sanded until the surface sits perfectly level with the rest of the panel, and then primer surfacer and paint go on top.

Think of it like plaster on a wall: it does not replace the bricks, it just leaves a smooth surface for painting. If the wall is crooked, the plaster will not straighten it.

When yes

• To fine-tune a panel that has already regained its shape, filling ripples of fractions of a millimetre that the eye picks up under low-angle light.
• On plastic parts such as bumpers, using specific flexible fillers that move with the material.
• To correct small marks after removing a dent or welding in a panel, always in a thin layer.

When no

• As a substitute for panel work. If a dent is filled instead of straightened, the filler layer is thick, heavy, can crack with vibration and temperature changes, and tends to show through over time.
• Over rust or loose paint. Filler does not stop corrosion: if there is rust underneath, it will keep spreading and eventually lift the repair.
• In structural or badly damaged areas. If a panel is torn or the structure is affected, the right answer is to repair or replace, not to fill.

How it should be applied

Good application starts before the tin is opened. The area is sanded, cleaned and degreased, and bare metal is given anti-corrosion protection. On plastics, an adhesion promoter goes on first, because without it filler comes away easily.

The filler is then mixed with its hardener in the ratio the manufacturer specifies. Too much or too little hardener and it will not cure properly; it can stain the paint or stay soft. It is spread with pressure to avoid trapping air, in thin crossed layers rather than one thick one. Finally it is sanded in several passes, from coarser to finer paper, until the surface is flawless to the touch and to the eye.

How to spot too much filler

If you are buying a used car or want to check an old repair, there are a few clues:

• Look along the panel under low-angle light: areas with heavy filler often show ripples or an outline.
• Look for fine cracks or small bubbles in the paint in one particular area.
• A magnet holds less firmly where there is a thick layer of filler (this does not work on aluminium or plastic panels).
• A paint thickness gauge, like the one used in a professional inspection, confirms it with actual readings.

Filler is not the problem. Using it instead of repairing is.

In our workshop we use it for what it is meant for: fine-tuning, not hiding. To see how we deal with a dent, have a look at our panel repair service. And if you are about to buy a car and wonder what might be under the paint, our pre-purchase inspection is worth a look.