Car Body Fillers

For a permanent and professional repair, the answer is no. Body filler must be applied directly to a properly prepared bare metal surface. Paint provides a weak, unstable foundation that will prevent the filler from bonding securely to the steel or aluminium underneath. Applying filler over paint will almost certainly lead to future peeling, bubbling, or cracking as the paint itself lifts away from the metal.

The correct procedure is to always sand the area down to clean, bare metal. We recommend using 80-grit sandpaper to remove all paint and create a coarse, scratched surface that gives the filler a strong mechanical grip for maximum adhesion and a truly durable, long-lasting repair.

Body filler should not be applied over primer. The correct and essential order of operations for a lasting repair is Metal > Body Filler > Primer > Paint.

Think of it this way: the filler’s job is to shape the repair on the bare metal, while the primer’s job is to seal the porous filler and the surrounding metal, creating a perfect, non-absorbent surface for the topcoat of paint to adhere to. Applying filler on top of primer reverses this process, creating a weak foundation. For tiny pinholes or minor scratches in the primer itself, a very thin layer of glazing putty is the correct product, but for all standard shaping and dent repair, the filler must go on the bare metal first.

No, body filler should be kept dry at all times. While modern body fillers are water-resistant once fully cured, they are also hygroscopic, meaning they can absorb moisture from the air.

If cured filler gets wet or absorbs humidity before being sealed, that moisture becomes trapped underneath the primer and paint. When the panel heats up in the sun, this trapped moisture can cause blistering, bubbles, and eventual paint failure from the inside out. For this reason, you must:

• Ensure the filler is fully cured before sanding.
• Always seal your sanded body filler with a quality automotive primer as soon as possible. The primer creates the essential waterproof barrier that protects your repair work.