How to prepare a car body before painting with paint

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The quality of a paint is determined before the first coat of colour is applied: it all comes down to the preparation.

A painter is judged by the shine of their work, but the real work is done beforehand: it is the preparation that determines how well the paint adheres and the final finish. A poorly prepared surface always shows – in the shine, the adhesion and how well it holds up over time. This tutorial walks you through the Carross workshop method step by step — washing, degreasing, sanding, filling, priming, masking — to achieve a clean, sound, flat and matt surface, ready to be painted without any nasty surprises.

Tech’Team tip

The tip that makes all the difference: Don’t try to hide a flaw with paint

Paint does not correct preparation flaws: it often makes them even more apparent. A bump, a scratch, a dent or a poorly sanded transition may seem inconspicuous on a matt surface, but become very visible after applying paint or a clearcoat. Paint only forms a thin layer. It is therefore no substitute for putty, sanding or a filling primer. The best approach is to check the surface before painting. If a defect remains visible after priming, you’ll need to go back: sand, apply more putty if necessary, reapply the primer and then check again.

If you can still see the defect before painting, it will generally be even more visible after painting. The finish is determined during the preparation stage, not when you’re spraying.

The essentials in a nutshell. Preparing a car body involves carrying out the following steps in order: washing, degreasing, progressive sanding, filling with putty, priming, then masking. Each stage determines the next; skipping or rushing any of them will result in defects in the final paint. A well-prepared surface is clean, sound, flat and perfectly matted. This is the method followed by our Carross technicians, from the initial wash to the final masking.

1. Assess the situation first and foremost

Sound surface, simple colour change — washing, degreasing, matt sanding and masking are sufficient
Scratches and minor dents — sanding, finishing putty, then a filling primer before painting
Bare metal or corrosion — treat the metal, apply epoxy primer, then reapply putty and primer
Plastic bumpers — specialised degreasing, matting, plastic primer before primer
Old paint that is blistered or chalking — strip the non-adhering area down to sound substrate before repainting

2. The equipment required

3. Step-by-step repair guide

Wash. Clean the part with water and shampoo to remove mud, salts and dirt before any abrasive treatment. Wash first because sanding a dirty surface drives particles deeper into the substrate and clogs the abrasive discs.
Degrease. Apply an anti-silicone degreaser using the ‘two-cloth’ method: a damp cloth to spread the product, followed by a dry cloth to wipe it off before it evaporates; otherwise, the greasy film will be reapplied. See ‘Degreasing a car body’.
Sand, then clean after sanding. Matt-finish and rough-sand according to the condition of the substrate, progressing step by step without ever skipping more than one grit. From the coarsest grit for rough sanding to the finest for the final matt finish; the choice of grit is detailed in ‘Car body sanding grits’.
After sanding:
- carefully blow away the dust with clean, dry air;
- vacuum or wipe away any residue;
- inspect the surface for defects or areas where the paint has not adhered properly;
- degrease the surface using the two-cloth method.
Apply putty. Fill dents and deep scratches with putty on a sound, dry surface that has been lightly sanded to ensure adhesion, applying in thin layers before it hardens. Once hard, sand smooth; for the right product, see which bodywork putty to choose.
Clean after sanding the putty, then apply primer. After sanding the putty:
- blow off and carefully remove all dust;
- remove any sanding residue from recesses and around the repair area;
- clean any marks left by handling with a compatible product.
Do not apply a large amount of degreaser to freshly sanded putty, as the putty is porous and may retain solvent. Use the two-cloth method, then wait for the solvent to evaporate completely.
The surface must be completely dry and free from dust before applying the primer.
Apply the appropriate primer for the substrate, allow it to dry for the specified drying time, then sand it to a fine finish; full details are given in the section on bodywork primers.
Remove dust and degrease the surface before masking. After sanding the primer:
- blow away any dust thoroughly;
- clean the surrounding areas;
- degrease using a product compatible with the paint system;
- wipe down using the two-cloth method;
- allow to dry completely.
Mask off the area and then carry out the final cleaning after masking. Protect any areas that are not to be painted properly. Apply the tape without stretching it excessively and seal the edges of the films and paper neatly; see ‘How to mask effectively’ for detailed instructions.
After masking:
- check that the edges are properly adhered;
- blow away any dust;
- apply a final coat of a compatible anti-silicone degreaser;
- use the two-cloth method;
- allow the surface to dry completely;
- wipe with a wiping pad just before painting.
Never start painting whilst the surface is damp, cold, greasy or dusty.

4. Common mistakes to avoid

Sanding a surface that has not been degreased: this embeds grease in the sanding marks
Going straight from putty to paint without a primer: insufficient adhesion and coverage
Skipping the final degreasing: greasy dust causes pitting
Skipping grit sizes when sanding: deep scratches show through the paint
Applying putty in a thick layer: it traps air and solvent, making the finish brittle
Masking with poor-quality tape: smudges and blurred edges
Reversing the order of the steps: this traps a defect or contamination beneath the next coat

5. How to choose your products

Scope of the job: the deeper the defect, the further back in the process you need to go (putty, epoxy)

Substrate: metal, plastic and painted surfaces require different products and primers

Desired finish: the more demanding the colour, the more important the primer and fine sanding become

Time available: a wet-on-wet application speeds up the process on sound surfaces, whilst a filler takes longer but improves surface flatness

Condition of the substrate: a sound substrate is matted down; a degraded substrate must be rebuilt before priming

Carross’s advice: if in doubt about the extent of the defect, it’s better to go back early in the process (putty, epoxy) than to try to correct it during the final sanding

6. FAQ

How do you properly prepare a car body before painting with paint?

By following this sequence: washing, drying, initial degreasing, sanding, dust removal and cleaning, filling if necessary, sanding the putty, dust removal and cleaning, primer, sanding the primer, dust removal and degreasing, masking, then final cleaning and degreasing before painting. The surface must always be sound, dry, clean and perfectly adhesive before the next product is applied. L’atelier Carross recommends never painting on a greasy, shiny or ungreased surface, nor skipping more than one grit when sanding.

How do you prepare putty for bodywork?

Apply the putty to a sound, dry and sanded substrate to ensure good adhesion, in thin layers, before it hardens. Once hard, sand it flat, gradually increasing the grit size step by step, then apply a primer. Details on product selection depending on the defect can be found in the putty guide.

How do you prepare a car body for spray paint?

The same principle applies, on a smaller scale: Wash and then dry the bodywork thoroughly. Degrease it, sand it down gradually, then remove any dust and clean the surface after each sanding. Repair any imperfections with putty if necessary, apply a suitable primer, then sand it down before cleaning and degreasing again.

Mask off the areas to be protected, carry out a final dusting and degreasing, and apply the paint in several thin layers, following the manufacturer’s instructions. Spray paint is even less forgiving of poorly prepared surfaces: the slightest trace of grease or residual shine is immediately visible.

How do you prepare a car body for painting on plastic?

Wash and use a degreaser on the part to remove any residue. Sand it down with a fine-grit sandpaper, then remove any dust and degrease it again after sanding. Next, apply a special plastic-bonding primer, followed by the primer. Without a suitable bonding agent, the paint may peel or flake off.

After sanding the primer, clean and degrease the surface once more before applying the paint.

Updated: 30/09/2026

Explore our preparation / painting / finishing section and our bodywork guides and tutorials. Any technical queries? The Carross Tech’Team: +33 (0)1 60 27 20 19.

Written and approved by Anthony M. Technical Manager at Carross View profile →