How to prepare a car body before painting with paint

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 receive the paint without any nasty surprises.

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 and then masking. Each stage determines the next; skipping or rushing any one 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 paint
• Bare metal or corrosion — treat the metal, apply epoxy primer, then apply further 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 rebuilding

2. Equipment required

• 150mm diameter abrasives — (1)
• Silicone-free bodywork degreaser — (1)
• Cloth for cleaning/degreasing — (1)
• Premium putty — (1)
• Easy-to-apply filling primer — (1)
• Premium masking tape — (quantity 1)
• Premium masking paper (quantity 1)
• Masking Film ( quantity 1)

3. Step-by-step repair

1. Wash. Clean the part with water and shampoo to remove mud, salt and dirt before any abrasive contact. Wash first because sanding a dirty surface drives particles deeper into the substrate and clogs the abrasive discs.
2. Degrease. Apply an anti-silicone degreaser using the ‘two-cloth’ method: a damp cloth to spread the product, and a dry cloth to wipe it off before it evaporates; otherwise, the greasy film will be reapplied. See [degreasing a car body](/degreasing-car-body-before-painting).
3. Sand --> Clean after sanding. Matt-finish and rough-sand according to the condition of the surface, progressing step by step without ever skipping more than one grit. From the coarsest for rough sanding to the finest for the final matt finish; the choice of grit is detailed in [car body sanding grit](/grain-poncage-carrosserie). After sanding: blow dust off thoroughly with clean, dry air; vacuum or wipe away any residue; inspect the surface for defects or areas with poor adhesion; degrease the surface using the two-cloth method.
4. Apply putty. Fill dents and deep scratches with putty on a sound, dry surface that has been scratched to ensure adhesion, applying thin layers before it hardens. Once set, sand flat; for the correct product, see [which putty to choose](/which-putty-to-choose).
5. Clean after sanding the putty, then apply primer. After sanding the putty: blow off and dust thoroughly; remove sanding residue from recesses and around the repair area; clean any handling marks 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 of dust before applying the primer. Apply the primer suitable for the substrate, allow it to dry for the specified drying time, then sand it to a fine finish; full details are given in [bodywork primers](/all-you-need-to-know-about-bodywork-primers).
6. Remove dust and degrease before masking. After sanding the primer: blow off the dust thoroughly; clean the surrounding areas; use a degreaser compatible with the paint system; wipe down using the two-cloth method; allow to evaporate completely.
7. Mask 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 the method in [How to perform effective masking](/reussir-masquage-avant-painting). After masking: check that the edges are properly sealed; 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; use a wiping pad just before painting. Never start painting whilst the surface is damp, cold, greasy or covered in dust.

4. Common mistakes to avoid

• Sanding an ungrease-removed surface: this embeds grease in the sanding scratches
• 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 will show through the paint
• Applying a thick layer of putty: this traps air and solvent, weakening the finish
• Masking with poor-quality tape: smudges and blurred edges
• Reversing the order of the steps: this traps a defect or contamination under the next coat

5. How to choose your products

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

Metal, plastic and paintedsurfaces do not require the same products or 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; a filler takes longer but improves flatness

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

Carross’s advice: if in doubt about the extent of the defect, it is better to go back earlier 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, putty 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 applying the next product. L’atelier Carross recommends never painting on a greasy, shiny or un-degreased 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 surface 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 it after each sanding. Repair any defects 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 degrease the part using a product suitable for plastic. Sand it down to a fine finish, then remove any dust and use a degreaser again after sanding. Next, apply a special plastic-bonding primer, followed by the primer. Without a suitable adhesion promoter, the paint may lift or peel. After sanding the primer, clean and degrease the surface once more before applying the paint.

Tech’team tip

Don’t try to hide a defect with paint. Paint does not correct preparation faults: it often makes them even more obvious. A bump, a scratch, a dent or a poorly sanded transition may seem inconspicuous on a matt surface, but can become very visible after applying paint or a clearcoat. Paint only forms a very thin layer. It is therefore no substitute for putty, sanding or a filling primer. The best practice is to check the surface before painting. If a defect remains visible after priming, you’ll need to go back: sand, fill again if necessary, prime once more, 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 preparation, not when you’re spraying.

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