Sanding metal: rust, welds, paint prep

Widespread rust and mill scale need stock removal: P40 to P60 on a random orbital sander with a large orbit, or a flap disc on the angle grinder. Sand until bare metal remains with no brown spots – otherwise rust pockets in the pores come back under the paint.

Then remove the deep scratches with P80. Where the machine cannot reach, non-woven abrasive by hand does the job.

Grind the weld flush with P60 first, without going into the surrounding panel – otherwise the metal gets thin and warps from the heat. Sand thin sheet in short bursts and let it cool. Break sharp edges with P120 so the paint does not pull away there.

For a good key, bare metal needs an even scratch pattern with P120 to P180; under primer surfacer and body filler it can be coarser, under direct-to-metal paint more like P240. Then degrease and prime promptly – bare steel will flash-rust overnight.

Stainless steel and aluminium that stay visible get a consistent scratch pattern with P240 to P400 or with non-woven abrasive – always in one direction. Use dedicated abrasives for stainless that have never touched ordinary steel: iron particles otherwise cause contamination rust.

The angle grinder with a flap disc removes material fastest – for welds, heavy rust and profiles. On thin body panels, however, it puts a lot of heat into a small spot; the panel distorts and is very hard to straighten again.

For flat areas, transitions and keying before primer, the random orbital sander is the better choice: it spreads the heat, leaves an even scratch pattern and, with dust extraction, works almost dust-free.

Safety glasses are a must, and with an angle grinder also ear protection and gloves. Sparks fly a long way: keep anything flammable well away, and cover glass, paint and chrome – hot particles burn into them.

Old coatings can contain lead or chromium. When sanding off old paint, use dust extraction and wear at least an FFP2 dust mask.