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Paint defects on bodywork

Defects in the field of automotive painting (but also motorcycle and bicycle painting) are common, even ubiquitous. They can be controlled through experience and mastery of the specific techniques used in painting.

It is important to know how to recognise defects and understand the causes of these paint defects. Special terms exist in the painter's jargon to describe each defect.

Most defects can be corrected without having to reapply the paint. Some defects are too problematic, however, and it is therefore necessary to reapply the paint system from the bottom layers, removing the layers that have already been applied.

The causes of defects can include, for example, poor preparation of the substrate, failure to observe drying times, excessive thickness, or even poor conditions in the workshop.

This guide aims to assist you in reliably assessing paint defects, whether you are a professional painter or a private individual and applicator: each defect is illustrated with its possible causes and recommendations for correction.

Summary

Refinishing — Paint defects (in alphabetical order)

Drips
Water stains
blurred, chalking, powdering
Solvent blistering, micro-bubbles, degassing, pinholes
Salt and pepper effect
Orange peel
Contours
Hardener migration in polyester filler
Sanding marks
Dust/impurities in the base coat
Dust/impurities in the clear coat
Poor adhesion between base coat and clear coat
Insufficient opacity
Condensation blisters or water bubbles
Frieze, paint wrinkling
Candy migration
Silicone crater, clear coat opening
Dull appearance or variation in gloss, loss of gloss
Poor adhesion to plastic parts
Micro-holes in polyester filler
Cracking, shrinkage
Loss of gloss in varnish during drying
Black or white marks on Chameleon paints

Identify, understand, prevent and correct defects in car body paint finishes

1. Drips

Possible causes :

Product viscosity too low, poor spraying technique, insufficient flash-off time between coats
Spray gun too close
Incorrect spray pressure
Product, substrate or booth temperature too low
Inappropriate choice of hardener or thinner too slow

Prevention :

Follow the recommendations in the technical data sheets.
Check that the spray gun is in good working order.
Heat the room and the products to approximately 20°C.
Use the correct hardener/thinner combination.
Keep the spray gun at a sufficient distance from the surface.

Correction :

Sand using a mini sanding block, then polish.
Or sand and repaint.
You can cut the edge of the drip using a blade before sanding.

2. Water stains

Cause :

Insufficiently dried varnish
Excessive film thickness, drying time too short
Unsuitable thinner
Incorrect hardener dosage
Incorrect hardener

Prevention :

Follow the instructions in the technical data sheets.

Correction :

Once completely dry, polish
If necessary, sand and repaint

3. Blurred, chalking, powdering

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Causes :

Excessive viscosity / poor technique / flash off, incorrect booth temperature
Angled position and excessive distance from the spray gun
Incorrect spray pressure
Inappropriate thinner

Prevention :

Adjust the product correctly.
Keep the spray gun parallel and perpendicular (90°) to the surface.
Keep the spray gun at a distance of 10 to 20 cm maximum from the surface.
Use a suitable spray gun assembly.
Use the recommended thinners.
Allow sufficient flash-off times.
Follow the recommendations in the technical data sheets.

Correction :

For classic base coats: use the ‘droplet’ technique before applying the varnish.
Once completely dry, sand and repaint.
For water-soluble base coats: apply an even finish to the wet base coat according to the technical data sheet.

4. Solvent bubbles, micro-bubbling, degassing, pinholes

Causes :

Partial drying of primer, filler or corners
Solvents or trapped air escaping during drying or released during recoating
Oven drying/baking without desolvation
Application of too many coats/too much thickness
Multiple layers of ‘millefeuille’ with or without sanding (chrome effects)
Incorrect viscosity, spray pressure, flash-off time/drying times
Excessive film thickness
Inappropriate spraying technique

Prevention :

Apply normal thicknesses (15-25°C), avoid hot temperatures.
Observe the recommended time intervals between coats.
Allow desolvation before baking.
Use a slow thinner at high temperatures.
Apply the recommended paint thickness.
Check the oven temperature.
Strictly follow the technical recommendations.
Avoid covering a chrome or silver paint system with new coats and opt for sanding or stripping the underlying coats.

Correction :

If within 24 hours: repaint without sanding.
Otherwise: sand with ultra-fine Scotch Brite.
Fill micro-holes with polyester filler or remove the finish and repaint.
Sand, prime and then repaint.

5. Pepper and Salt Effect

Causes :

On a water-soluble base, too much moisture on a metallic base causes small black spots in light colours.

Prevention :

Observe the dosages, pressure and spraying technique for the base coat.

Correction :

This phenomenon often occurs after varnishing: sand and rework the base coat + varnish.

6. Orange peel

Peau d'orange

Causes :

Incorrect pressure, viscosity, technique or application temperature
Inappropriate thinners
Poorly sanded surface preparation
Incorrect spray gun settings and position
Excessively high temperatures

Prevention :

Follow the technical recommendations.
Check the ambient temperature or choose a cooler time of day (morning).
Check the position and distance of the spray gun from the surface (10-15 cm and 90° angle).
Prepare and sand properly.
Use the recommended thinners, especially slow thinners in summer.
Avoid painting in extreme conditions (+25°C).

Correction :

Sand and revarnish
Or sand and then polish

7. Contours related to solvent penetration

Contours liés à la pénétration du solvant

Causes :

Area sanded down to the substrate, not insulated
Area sanded down through several layers
Inappropriate insulation/primer
Insufficient drying of the substrate

Prevention :

Test the substrate with a solvent test.
Insulate properly with 2K primer in several thin layers.
Do not sand through the insulating layer.

Correction :

Dry the varnish completely
Sand the area, seal it, then repaint

8. Migration (bleeding) of the hardener into the polyester filler

Causes :

Excess hardener
Insufficient mixing
Non-insulated filler

Prevention :

Use precise measurements
Weigh the hardener
Mix thoroughly
Use suitable insulation

Correction :

Sand, insulate with sprayable polyester filler, prime and then repaint.

9. Scratches caused by sanding

Causes :

Paper grain too abrasive
Sanding of still-soft filler
Insufficient insulation before applying the top coat

Prevention :

Use the appropriate grit (P500 for primers and varnishes)
Isolate the repaired area with 2K primer
Ensure that the materials are completely dry

Correction :

Sand correctly
If necessary, insulate or remove the substrate and repaint

10. Dust/impurities in the base (two-coat paint to be varnished)

Causes :

Dust ingress into soft film (ambient air or unfiltered base)

Prevention :

Keep the cabin clean.
Blow dust off floors/walls/corners at each stage of preparation.
Wear lint-free overalls.
Filter/tin the paint.

Correction :

Sand with P500 dry or wet sandpaper, then touch up the base on the affected areas.

11. Dust/impurities in the varnish.

Poussières dans le vernis

Causes :

After application, the dust adheres to the film while it is still liquid.

Prevention :

Maintain strict cleanliness in the cabin.
Blow dust off the floors and corners of the cabin before varnishing.
Wear lint-free overalls.

Correction :

Sand/polish if the imperfection is superficial.
Otherwise, reapply the varnish after sanding (P500 / P800).

12. Poor adhesion between base coat and varnish

Causes :

Base coat or varnish too thick.
Recoating beyond the wet-on-wet application time.
Recoating after drying and sanding too finely.
Intermediate/final flash-off times too long.
Incorrect varnish/hardener ratio.
Inappropriate varnish/thinner combination, system too fast.

Prevention :

Follow the instructions in the technical data sheet regarding sanding grit (P500) and recoating time (generally 30 minutes for a solvent-based product and 24 hours for a water-based product)
Observe the thickness of the base coat and varnish
Observe the flash-off times
Select and measure out the varnish, hardener and thinner correctly according to the technical data sheet

Correction :

Sanding and repainting

13. Insufficient opacity

Causes :

Non-uniform substrate (different background colours)
Semi-opaque or semi-transparent paint
Coat thickness too thin
Excessive thinner

Prevention :

Ensure a smooth, even surface (primer recommended for colour)
Apply a sufficient thickness
Avoid using too much thinner

Correction :

Sand with P500, then apply the required number of coats of paint.

14. Moisture blisters or water bubbles

Causes :

Sanding water residue in corners
Contaminated compressed air
Insufficient insulation of polyesters
High humidity
Poorly flash-offed or poorly polymerised water-soluble products
Salt residue

Prevention :

Remove external fittings.
Blow and dry thoroughly.
Check compressed air (maintain pipes and air compressor).
Use or double air filters.
Observe flash-off/cooking times for water-soluble products.
Clean according to instructions.

Correction :

Remove any contaminated finish by sanding with P500 sandpaper and repaint.

15. Frieze, paint wrinkling

Possible causes

Chemical reaction between two incompatible layers.
Presence of undissolved degreaser in the substrate.
Application of a layer that is too thick.
Coating of a substrate that is not yet completely dry.
Use of a wet-on-wet system with an unsuitable hardener/thinner combination.

Prevention

Avoid applying thick layers.
Ensure that all products used are compatible and belong to the same system, and only use professional products.
After applying a degreaser, remove it immediately with a clean cloth.
Observe the recommended flash-off and drying times.
Use the thinners and hardeners specified by the manufacturer.

Correction

Remove the coating completely down to the bare metal.
Any other partial solution is unstable and not guaranteed.

16. Candy Migration

Possible causes

Mixing Candy into a single-component base
Excessive dosage of Candy
Using light colours on Candy
Recoating after insufficient drying time

Prevention

Mix Candy in two-component bases.
Cover a single-component Candy base with a two-component varnish.
Observe the maximum Candy dosage of 6% in bases and varnishes.
Allow the paint to dry completely before recoating.
Avoid applying light colours (e.g. white) over a Candy base.

Correction

Apply a two-component varnish.
Then repaint with the colour to cover the colour migration, then varnish.

17. Silicone crater, varnish opening

Possible causes

Presence of contaminants in the substrate
Poor substrate preparation
Presence of surface tension
Product contamination
Use of incompatible or excessive additives in the paint or varnish

Prevention

Eliminate the presence of contaminants in the workshop (silicone, waxes, etc.)
Use a professional silicone degreaser before painting and varnishing
Clean and rinse with water and a surfactant
Apply one or two very thin ‘flash coats’ (bonding coats)
Use an anti-silicone additive

Correction

Sand the surface and reapply the paint or varnish.

18. Dull appearance or variation in shine, loss of shine

Possible causes

Excessive film thickness or ambient humidity too high.
Substrate sensitivity to solvents.
Hardener incorrectly dosed or contaminated.
Unsuitable solvents.
Insufficient air circulation in the booth.
Curing interrupted.
Varnish applied too quickly.
Polishing too early after drying.
Poor spraying/spray gun position.

Prevention

Follow the instructions in the technical data sheets.
Paint at a temperature between 15 and 25°C.
Hold the spray gun at a 90° angle to the surface and maintain a steady, regular speed and distance. Apply an even quantity/coverage.
Seal the hardener containers tightly after use.
Ensure good air circulation in the oven.
Do not interrupt the baking cycle.

Correction

If the defect is minor: sanding and polishing.
If necessary: complete sanding and total refinishing.

19. Poor adhesion to plastic parts

Possible causes

Insufficient preparation of the substrate
Insufficient cleaning or drying (tempering)
Absence of adhesion promoter or primer unsuitable for plastic
Type of plastic
Paint system unsuitable for plastic substrates
Paint too thick and/or not flexible

Prevention

Temper or heat plastic parts before cleaning and applying primer.
Clean and degrease thoroughly.
Wait for solvents to evaporate completely.
Use an adhesion promoter suitable for the type of plastic.
Apply the techniques and instructions for preparing plastics (flaming, sanding with abrasive sponges and thinners, anti-static application).
Limit the thickness of paint and varnish layers.
Add a flexibilising additive.

Correction

Sand with an abrasive sponge, degrease and repaint.
If adhesion is severely compromised: remove the coating and refinish.

20. Micro-holes in polyester filler

Possible causes

Incomplete drying of the substrate.
Lack of insulation with a 2K primer.
Superficial sanding that does not remove pores.

Prevention

Ensure that all preparation materials are completely dry.
Sand the pores thoroughly or reapply a coat of filler.
Finish with a fine finishing filler.
Mix the filler thoroughly with the hardener to prevent air bubbles from forming.
Apply the filler with a spatula perpendicular to the surface to limit the appearance of pores.

Correction

Combler les micro-trous à l’aide d’un mastic pulvérisable.
Poncer la zone puis procéder à une nouvelle finition

21. Cracks, shrinkage

Possible causes

Use of incompatible paints in the same system.
Recoating without sanding on a dry surface.
Application of too many coats/too thick a layer.
Failure to observe the flash-off interval between coats.
Product with high solids content, poorly homogenised.
Excessive heat.

Prevention

Apply to a sanded surface with sufficient abrasive grain.
Apply the thickness recommended in the technical data sheet, without excess.
Allow sufficient flash-off time between coats.
Check that the product has been properly stirred/homogenised (avoid excess matting or fluorescent load).
Check the ambient application temperature (15-25°C).

Correction

Sufficient sanding and/or priming, sanding with P500 grit sandpaper, then repainting.

22. Loss of gloss in the varnish during drying

Possible causes

Temperature too high
Oven drying or IR baking too fast (without flash-off)
Drying in a humid environment
Use of an incompatible thinner

Prevention

Allow evaporation time (flash off) as specified in the technical data sheet before oven drying/baking or IR lamp treatment.
During drying, monitor the temperature and humidity – ensure a dry environment.

Correction

Polishing
Sanding and refinishing

23. Black or white marks on chameleon and pearlescent paints, fish-eye effect

Possible causes

Incorrect application, uneven and/or insufficient application (dark marks)
Application of thick layers (white marks)
Insufficient interval between layers (white marks)
Excessive temperature
Incorrect spray gun position (chalking)

Prevention

Apply thin coats, avoid thick coats.
Apply coats until the base is covered.
During drying, monitor the temperature and humidity – ensure a dry environment.
Check that the product has been properly stirred/homogenised. (pigment may have settled in the pot).
Apply at moderate temperatures (15-25°C) and use a suitable thinner.

Correction of imperfections (black marks)