Sanding plastic: preparing bumpers properly

Plastic conducts heat poorly. At high speed and with a lot of pressure the surface softens, the grain clogs, and instead of sanding dust you get fluff and smears. So: a random orbital sander with a small orbit at low speed, light pressure, and change the disc once more rather than once less.

New parts often carry release agent from the mould. Wipe them with plastic cleaner before the first sanding – otherwise the abrasive just rubs it into the surface.

Smooth deep scratches and moulding flash with P180, then remove the scratches with P240 and P320. Coarser than P180 is rarely needed on plastic and leaves scratches you will see in the paint later. Damage that needs filling calls for a plastic filler – normal polyester body filler is too hard and cracks when the part flexes.

For new paint, the whole surface must be matt without a single shiny spot: P400 on the random orbital sander on flat areas, and grey non-woven abrasive in edges, swage lines and honeycomb grilles – it follows the contour and does not sand through.

Then clean thoroughly, including with an anti-static cleaner, because plastic attracts dust. On bare plastic, adhesion promoter goes on before the primer; without it, the paint flakes off at the first stone chip.

Visible parts that will not be painted: fine-sand with a film-backed disc or foam pad from P600 to P1000, preferably wet. The even scratch pattern is the basis for polishing.