How to sand a body panel

A good sanding job isn’t visible: you can feel it to the touch – smooth and even, with no marks, ready for the colour.

Sanding a surface isn’t about rubbing it until it shines: it’s about removing just the right amount of material, matting the surface and gradually increasing the grit size to smooth out any scratches. If done incorrectly, sanding leaves marks beneath the paint that become visible in the first low-angle light. When done properly, it produces a sound, flat and even surface to which primer and paint will adhere flawlessly. This tutorial details the techniques used at the Carross workshop, whether dry or wet sanding, step by step. As for the precise choice of grit sizes layer by layer, I’ll refer you to the dedicated guide rather than explaining it again here.

The essentials in a nutshell. Sanding a part involves rough-sanding and then fine-sanding the surface, gradually increasing the grit size from the coarsest to the finest, without ever skipping more than one grit at a time. We use a sanding block or a sanding machine to ensure the surface remains flat, sanding either dry or wet depending on the layer being worked on, until a uniform matt finish is achieved with no visible scratches. Checking the work under raking light confirms the result. This is the standard procedure our Carross technicians follow systematically before moving on to the next stage.

1. Diagnose first and foremost

• Sound existing paintwork to be recoated — sanding to a fine-grit matt finish to create a bond, without exposing the metal
• Localised scratches and defects — rough-sand with a medium-grit sandpaper on the affected area only, then refine and fill if necessary
• Paint runs or excess paint — dry-sand with a medium-grit sandpaper, flat against the block, then gradually smooth
• Putty to be levelled — rough-sand the shape, then increase the grit size up to the finish before priming
• Bare sheet metal or exposed metal — sand to mattify thesurface and create a bond; protect immediately with a primer to prevent oxidation
• Filling primer requiring sanding down — once completely dry, sand progressively with a dry sander to level the surface without sanding through to the underlying layer

2. Equipment required

• EG150 discs, P80 to P1000 — (quantity 1)
• Sanding blocks for 150 mm diameter abrasive discs — (quantity 1)
• Silicone-free bodywork degreaser — (1)
• Degreasing cloths ( 1)

3. Step-by-step repair guide

1. Degrease first. Clean the part with the silicone-free degreaser to prevent grease from becoming embedded in the scratches during sanding.
2. Identify the surface layer. Determine what you are sanding (sound paint, putty, primer, bare metal) and what layer will follow: this determines the starting and finishing grit sizes.
3. Sand down gradually. Tackle the defect with the appropriate abrasive, one that is sufficiently aggressive for the job at hand, without using an unnecessarily coarse grit. Sand by hand or with a suitable sanding machine, without applying excessive pressure, to ensure the surface remains flat.
4. Gradually increase the grit size. Refine the surface step by step, never skipping more than one grit, and cross-sanding to remove any scratches left by the previous grit.
5. Choose between dry or wet sanding. Opt for dry sanding for putty, primer and pre-paint preparation. Adapt the grit size at each stage to suit the substrate and the subsequent coat. Wet sanding may be reserved for certain fine finishes, if the process allows, ensuring the surface is completely dry before applying any products.
6. Check the surface. Run your hand flat across the surface and inspect the piece under low-angle light to detect any dents, bumps or residual scratches. Re-sand locally with the appropriate grit if necessary.
7. Achieve a uniform matt finish. Finish with the grit size recommended for the next coat. The surface must have an even matt finish, with no shiny patches or deep scratches. Dust and degrease one last time using a suitable product before applying the primer or paint.

4. Common mistakes to avoid

• Skipping one or more grits: coarse scratches remain and may reappear under the paint.
• Sanding with your fingertips: pressure is concentrated on small areas, which can locally indent the surface and create marks or ripples. Use a suitable sanding block to distribute the pressure.
• Applying excessive pressure to the sanding machine: this heats the surface, clogs the abrasives and can create localised dents or sanding marks.
• Sanding water-based putty: as putty is often porous, it can absorb water. This can subsequently lead to adhesion problems or cause defects beneath the primer.
• Ignoring the effect of raking light during inspection: some dents, bumps or residual scratches may go unnoticed until the clearcoat is applied.
• Sanding through an edge or a layer by applying too much pressure in the same spot: this risks exposing the metal sheet or an undercoat.
• Failure to remove residue properly after sanding: blow the surface clean thoroughly, particularly in joints, edges and corners, then use a suitable degreaser. Use clean, lint-free cloths, and avoid touching the prepared surface with bare hands before applying the product.

5. How to choose your products

The various stages of the project – rough sanding, preparation and finishing – do not require the same starting or finishing grit.

Dry or wet sanding: opt for dry sanding for rough sanding, puttying and preparation before painting. Wet sanding may be reserved for certain fine finishes, only if the process and products allow it. The surface must then be thoroughly dried.

Abrasive support: use a block or a firm pad on flat surfaces, and a flexible pad to follow the contours without distorting them.

The next coat: the thinner the coat to be applied – such as a basecoat or clearcoat – the finer the finishing grit must be.

Choosing the abrasive: Abrasive discs and abrasive belts are now the preferred choice for dry sanding, using a sanding machine connected to a dust extraction system. They also work very well with sanding blocks suitable for manual use.For a water-based finish, use an abrasive specifically designed for this process.

Carross’s recommendation: if in doubt about which grit to use, it is better to err on the side of caution and proceed in small increments rather than skipping several grits or applying excessive pressure.

6. FAQs

Should you sand between two coats of bodywork paint?

Generally speaking, no – not between two basecoats applied at the same time: allow the specified flash-off time and then apply the next coat. Intermediate sanding is only necessary to correct a defect that has appeared – such as dust, runs or overspray. Always follow the technical data sheet for the paint system, which takes precedence over any general rule.

How do you sand putty?

Dry sanding only. Rough-sand the surface with a medium-grit sandpaper to level the putty, then gradually increase the grit size, without skipping any grades, up to the required finishing grit before applying the primer. Avoid using water: the putty is porous and allowing it to become saturated would compromise the adhesion of the next coat. The Carross workshop also recommends always checking by touch that the putty is completely dry before applying the abrasives.

How do you sand a car body before painting with paint?

Sand down the existing sound paintwork with a fine-grit sandpaper to create a bond, without sanding through to the metal. Where there are defects, roughen the area locally, fill with body filler if necessary, then work your way up through the grit sizes to the finish. Finish by degreasing the surface before priming or painting.

Do you need to sand the paint before applying clearcoat to the car body?

No, if the base coat is recent and part of the same paint system. The clearcoat is applied directly over the base coat, after the flash-off time specified in the manufacturer’s technical data sheet. If the base coat has a defect, sand only the affected area, then reapply a thin layer of basecoat to cover the scratches. In the case of old paint or clearcoat, the surface must be thoroughly degreased, then sanded to create an even bonding surface for the new system. Use an abrasive suitable for the condition of the substrate. After removing dust and carrying out a final degreasing, apply the primer or base coat in accordance with the manufacturer’s technical data sheet before applying the clearcoat.

What grit should be used for sanding car body paint?

This depends on the intended layer and what comes next. The complete process, from bare metal to polishing, is detailed in the guide [car body sanding grit](/grain-poncage-carrosserie). In summary: the thinner the next layer, the finer the finish required, to ensure a smooth bond with no visible scratches.

Tech’team tip

Use a dry Guide Coat A dry Guide Coat is a thin layer of coloured product applied to the surface before starting a new sanding stage. It helps you visualise imperfections and monitor your progress. When applied to a surface that has been puttyed or primed, the guide settles into scratches, hollows and lower areas. When sanding with the appropriate grit, the highest areas lighten first, whilst the guide remains visible in the scratches left by the previous grit or in any defects that are still present. You must continue sanding until you achieve a uniform surface, with no lingering traces of the guide. This helps to ensure that the scratches left by the previous grit have been completely removed. If any marks remain visible, do not move on to the next grit: as this is finer, it may simply polish over the scratches without removing them. The guide also helps to prevent sanding too deeply. If you reach the underlying layer whilst the guide is still visible in some places, this usually indicates a dip or an irregularity. You should then reassess the flatness of the surface rather than continuing to sand in that specific area. The best result does not come from more vigorous sanding, but from careful control of the surface, a suitable abrasive and a steady progression through the grit sizes.

Explore our preparation / painting / finishing section and our bodywork guides and tutorials. Got a technical query? Contact the Carross Tech’Team: +33 (0)1 60 27 20 19.