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The foundation of a flawless paint finish is laid in the sanding steps hidden beneath the paint. The wrong grit or a skipped step comes back as scratches showing through the paint, wavy surfaces and lost labour hours. In this guide we summarise which abrasive to use, and how, at every stage from filler to clear coat.
What is the P grit system?
The P numbers on abrasives are based on the FEPA P scale (ISO 6344), which defines the size of the abrasive grains and is widely used in Europe. The higher the number, the smaller the grain – in other words, the finer the abrasive:
- P40–P120: Removes material quickly and leaves deep scratches; for rough preparation and shaping filler.
- P150–P320: Completes the shaping and refines the scratches.
- P400–P1000: Prepares the primer surface for paint.
- P1500–P3000: Preparing blend zones and removing clear coat defects.
Numbers without the letter P may belong to a different scale; particularly with fine grits, the same number does not mean the same fineness. Always go by the P scale when buying abrasives and in workshop instructions.
A typical sanding sequence from filler to clear coat
| Operation | Typical grit | Note |
|---|---|---|
| Removing old paint, rough preparation | P80 | Removes old paint and keys the surface |
| Rough shaping of filler | P80–P120 | Long, flat block or 5 mm orbital |
| Fine shaping of filler | P150–P240 | Check flatness with a guide coat |
| Feathering edges before priming | P240–P320 | Feathers out filler and paint edges |
| Primer, dry sanding | P400–P500 | 3 mm or 2.5 mm orbital |
| Primer, wet sanding | P800–P1000 | By hand with a block |
| Local preparation before basecoat | P1000–P1500 / grey scuff pad | Mattes the blend zone |
| Removing clear coat defects | P1500–P3000 + polishing | On the defective area only |
Body fillers such as ZN-110 Universal Polyester Filler are shaped with coarse and medium grits, whereas ZN-130 Soft Finishing Filler, which is used to seal pores and fine scratches, is typically finished with P180–P240. The values in the table are typical ranges; because the final sanding grade before basecoat in particular can vary from system to system, follow the Technical Data Sheets (TDS) of the products you use. You can also take a look at our application systems page for filler, primer and clear coat combinations.
The 100-grit rule: don't skip steps
Each grit is used to remove the scratches left by the one before it. The common rule in practice is not to jump by more than 100 grit numbers at the next step when working with coarse and medium grits. For example, going straight from P80 to P240 removes the deep P80 scratches only after a very long time; some of these scratches often remain on the surface and reappear beneath the primer or paint.
Example sequences:
- Filler: P80 → P120 → P180 → P240
- Feathering edges: P240 → P320
- Primer (dry): P320 → P400 → P500
With fine grits above P400, the difference in grain size becomes smaller, so larger numerical steps (for example P1000 → P1500 → P2000) are accepted. But the logic stays the same: do not move on to the next grit until the scratches from the previous one have completely disappeared. When sanding by hand, changing the sanding direction to a cross angle with each new grit makes any scratches left in the old direction easy to spot.
Dry sanding, wet sanding and scuff pads
In today's workshops, dry sanding with dust extraction has become standard: it is fast, leaves no water on the surface and the result is easy to check. Wet sanding, on the other hand, gives a finer surface, particularly on primer and clear coat.
- Filler: Polyester fillers are always dry sanded. Filler can absorb water, and trapped moisture later leads to blistering and corrosion.
- Primer: P400–P500 is typically used for dry sanding and P800–P1000 for wet sanding. After wet sanding, wait until the water in edges and gaps has dried completely; bare metal spots exposed during sanding can rust quickly, so dry them and cover them with a suitable primer.
- Clear coat: To remove defects, sand wet, or dry with a soft interface pad, using P1500–P3000, then polish.
2K primers such as ZN-210 4+1 Acrylic Primer Filler or ZN-250 Joker Multi-Purpose Primer must not be sanded until the drying specified in the TDS is complete. Primer that has not dried sufficiently clogs the abrasive and later sinks back, revealing sanding marks.
Matting with scuff pads
On door edges, contours and bumper corners that a sanding disc cannot reach, a grey scuff pad used together with a scuffing paste gives an even matt finish without leaving deep scratches. Scuff pads are the preferred choice for preparing the basecoat blend zone and adjacent panels, especially in spot repairs. Do not leave any glossy spots on the surface; areas that remain glossy lead to poor adhesion.
See the invisible with a guide coat
A guide coat is a dry, usually dark-coloured powder that is applied to the surface in a thin layer. During sanding, the powder on high spots is quickly removed, while it remains in low areas, pores and deep scratches. This makes defects visible that cannot be detected by eye or by touch.
- Reapply the guide coat before every change of grit; when it has disappeared completely, the scratches from the previous grit have been removed.
- Use it to find low spots and pinholes in filler, and waviness and remaining scratches in primer.
Tip: Don't chase stubborn guide coat spots with abrasive. These spots are usually low areas; filling them with a thin layer of finishing filler saves time and prevents the surrounding surface from being thinned down unnecessarily.
Hand block or orbital (DA) sander?
A hand block is indispensable for shaping filler, especially on flat surfaces. A long, rigid block cuts down the high spots and levels the panel; flexible blocks conform to curved surfaces. The block should be long enough to extend beyond the repair area in both directions; a short block follows the low spots and leaves a wavy surface. Never sand filler with just your fingers or the palm of your hand, as this leaves grooves in the surface. Working the filler with crosswise (X) strokes improves flatness.
On orbital (DA) sanders, the orbit diameter determines how aggressively the machine works:
- 5 mm: Aggressive and fast; for shaping filler and coarse grits (P80–P240).
- 3 mm: General purpose; for feathering edges and sanding primer (P240–P500).
- 2.5 mm: Fine, even scratch pattern; for final primer sanding, preparation before basecoat and clear coat (P500 and above).
Keep the pad flat on the surface; tilting the machine leaves deep rings and waviness. Use a soft interface pad on curved surfaces and with fine grits, do not press too hard, and replace worn or clogged discs in good time.
Dust extraction: for quality and for health
Sanding dust is not only a health risk but also a quality problem. Dust trapped between the disc and the surface clumps together and cuts deep scratches, shortens disc life and, when carried into the booth, leads to dust defects.
- Use discs whose hole pattern matches the machine's pad, and work with a mobile or central dust extraction system.
- Even with extraction, wear a particle filter mask (P2/P3) and safety goggles when dry sanding.
- After sanding, blow the surface clean with compressed air and wipe it down with a suitable surface cleaner; use a tack cloth immediately before painting.
Summary
Choosing the right abrasive means starting with a coarse grit suited to the job, progressing without skipping steps in line with the 100-grit rule, always dry sanding filler and checking every step with a guide coat. On orbital sanders, choose the orbit diameter to match the fineness of the work, don't neglect dust extraction, and follow the TDS of the product you are using for the final sanding grade.