What Sandpaper Grit Should I Use Before Painting?

Short answer

The correct sanding grit depends on the surface you are preparing and on the next layer that will be applied. It is not true that a rougher sanding mark always gives better adhesion. It is also not true that finer sanding automatically means better preparation.

If the surface is sanded too coarsely, deep sanding marks can later show through the following layers as visible edges, sink marks or outlines. If the surface is sanded too finely or almost polished, the next layer may have poorer adhesion.

For PMcolours effect paints, which are prepared similarly to metallic paints, we generally recommend preparing the surface under the paint with approximately P800. However, you must always consider whether you are sanding metal, old paint, plastic, body filler, epoxy primer or filler primer.

Why is there no universal sanding grit?

Every material sands differently. The same grit will create a different surface on bare steel, aluminium, plastic, old paint or filler primer. The sanding method, tool and quality of the abrasive also play an important role.

Hand sanding leaves a different surface than an orbital sander. That is why it is not possible to say that P180, P400 or P800 is always correct for everything. The correct grit is chosen according to the substrate and the next layer.

Coarser sanding does not automatically mean better adhesion

A common mistake is thinking that the rougher the surface, the better the next material will hold. Some layers do need sufficient mechanical adhesion, but sanding marks that are too deep can create problems later.

Deep scratches can later sink or show through filler primer, base colour, effect paint or clear coat. On a repaired part, visible edges, outlines or uneven areas can appear even if the surface looked acceptable before painting.

Finer sanding does not always mean better work

The opposite mistake is also common. If the surface is prepared too finely, it can become too smooth for the next layer. The new material may then not achieve proper mechanical adhesion.

Correct preparation is therefore not about using the finest possible abrasive, but about using the correct grit for the specific material and the next layer. Ideally, always follow the technical data sheet for the specific product.

Typical grits before applying body filler

Before applying body filler, the surface must be rough enough for adhesion, but not unnecessarily coarse in the wrong areas. Otherwise, visible transitions and sink marks can appear later.

Surface	Typical grit
Steel	P80–P120
Aluminium	P120–P180
Old paint	P180–P240
Plastic	P240–P320

The transition from metal to original paint should not be sanded with one grit across everything. Metal and old paint may require different preparation, otherwise visible outlines or later sinking of the repair can occur.

Typical grits before primer

Before primer, epoxy primer or plastic primer, the grit is selected according to the material. The goal is to prepare a surface that gives the primer good adhesion without creating sanding marks that are too deep.

Surface	Typical grit
Steel	P120–P180
Aluminium	P180–P240
New plastics	P400–P500 or Scotch-Brite

Typical grits before filler primer

Filler primer is used to even and level the surface. For it to adhere correctly, the substrate must be prepared properly. The grit varies depending on whether the surface is OEM primer, epoxy primer, polyester filler, old paint or plastic.

Surface	Typical grit
OEM primer	P320–P400
Epoxy primer	P180–P400
Polyester body fillers	P180–P400
Old paint on metal	P180–P400
Old paint on plastic	P240–P400

Sanding filler primer before topcoat

After curing, filler primer is sanded so the surface is flat, even and ready for the topcoat. At this stage it is important not to leave deep sanding marks that could show through the final layer.

Topcoat material	Typical grit
Solid / UNI colours	P360–P600
Metallic paints and basecoats	P400–P800
PMcolours effect paints	we generally recommend P800
Matting clear coat	P800–P2000
Blending into clear coat	approx. P3000

PMcolours effect paints and P800

Our effect paints, such as Chameleon / Flip-Flop, Candy, Pearl, Ghost Pearl, Hologram and Glitter, are sensitive topcoats in terms of substrate preparation, similar to metallic colours. For this reason, we generally recommend preparing the surface under the paint with P800.

The surface must be even and free from deep sanding marks, dust, grease and contamination. With effect paints, substrate defects can easily show in the final appearance because the effect, light reflection and shade depend strongly on the quality and evenness of the base.

Should base colour or effect paint be sanded?

The base colour before the effect layer is normally not sanded if it has been applied correctly and has no defects. The effect paint itself is also normally not sanded before clear coat.

Sanding the base or effect layer can disturb metallic particles, pearl pigment, colour shade or evenness of the effect. Final smoothness, gloss and protection should be created by correctly applied clear coat.

Be careful on edges and body lines

When sanding, be especially careful on edges, body lines and sharp transitions. These areas are easy to sand through down to another material, through the base layer, filler primer or old paint.

A sand-through area must then be treated correctly again. If bare metal appears, it should be treated with a suitable primer, preferably epoxy primer, and then the correct layer system should be continued.

What to do after sanding?

After sanding, it is not enough to simply start painting. The surface must be thoroughly cleaned of sanding dust, degreased and dry. Sanding dust can remain in edges, holes, pores or gaps and later cause paint defects.

• remove sanding dust,
• degrease the surface with a suitable degreaser,
• allow the surface to flash off completely,
• do not touch the surface with bare hands after degreasing,
• shortly before painting, an anti-static tack cloth can be used to remove fine dust.

Summary

• The correct sanding grit is chosen according to the substrate and the next layer.
• Coarser sanding does not automatically mean better adhesion.
• Sanding marks that are too deep can show through later layers.
• A surface prepared too finely can reduce adhesion.
• For PMcolours effect paints, we generally recommend preparing the surface with P800.
• Base colour and effect paint are normally not sanded before the next layer unless a defect needs to be repaired.
• After sanding, the surface must be cleaned, degreased and allowed to flash off.
• It is always best to follow the technical data sheet for the specific product.