Guangzhou Jinwei Chemical Co., Ltd

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Paint Defects Manual ( VIII )
- Jun 23, 2018 -

Paint Defects Manual (8)



36.Sanding Marks

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DEFINITION

Sanding swirl marks in the substrate show as lines in the top-coat.

CAUSES

[1]The substrate was sanded with an abrasive that was too coarse. The depth of the scratch is too deep for the subsequent products to fill and hide. 

[2]The correct drying times for the primer or surfacer coats were not observed correctly. The swirl marks from the sanding operation are clearly visible due to the swelling of the primer at the time of painting and the shrinkage on drying. 

[3]Filler coats were applied too thin to cover the sanding marks in the substrate. 

[4]Poor sanding technique and/or sanding machine.

HOW TO AVOID

[1]Use specified grade of sanding disc and/or paper (e.g. for stoppers use P80/P150, primer filler/filler P240 – refer to relevant tech data sheets). 

[2]Dry as per specification. 

[3]Observe specified film builds. 

[4]Always place sanding machine on surface of paintwork before starting the machine. For dry sanding of primer/filler coats, the eccentric stroke should not be greater than 5mm.

HOW TO REPAIR

Sand out the scratches and refinish with correct filler coat and/or top-coats as necessary.


37.Spray dust/Over Spray

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DEFINITION

Fine dry atomised spray droplets from the painting process stuck to the surface, or droplets that have not been absorbed by the paint film.

CAUSES

[1]Poor spray absorption due to the incorrect hardener and/or thinner for the painting conditions or the size of the object which is being sprayed. 

[2]Inadequate overlap of spray pattern when spraying.

HOW TO AVOID

[1]Select the correct choice of hardener and thinner to suit the temperature of the workshop and the size of the object/vehicle which is to be painted. (see temperature table). 

[2]Ensure adequate overlap of previous gun strokes when spraying. 

HOW TO REPAIR

Polish out with a fine polishing paste a high gloss polish.


38.Spray dust Clusters

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DEFINITION

Paint dust particles from the spray environment in the new paint surface.

CAUSES

[1]Impurities due to paint residue/encrustation of the spray gun, paint hoses and clothing. 

[2]Paint clusters (multi-coloured particles) from the spray booth fall into the wet paintwork. These particles often form on the roof of the spray booth if the air changes are poor or the filters are dirty. 

[3]Paint spray contamination from adjacent paint jobs.

HOW TO AVOID

[1]Regularly clean spray guns, air hoses and clothing. 

[2]Regular maintenance of the spray booth and filter systems and ensure the booth manufacturers set the airflow correctly. 

[3]Keep jobs separate and masked from adjacent jobs.

HOW TO REPAIR

Minor faults may be rectified by polishing with a fine polishing paste, and finishing with a high gloss polish. Severe faults will require sanding back and repainting.


39.Stone Chips

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DEFINITION

Mechanical damage to the coating due to projected small stones/grit (e.g. chippings).

CAUSES

Stones are flung onto the paintwork at varying levels of energy (velocity/size). Depending on the impact force, the top layers and also the other paint layers underneath can be damaged down to the substrate. At the points of damage, humidity will penetrate the coating and lead to corrosion creepback and flaking.

HOW TO AVOID

There is no total protection against stone chips. Areas most greatly at risk can be given preventive treatment of additional anti-chip and underbody protection coatings for extra protection to avoid corrosion of the substrate. (e.g. appropriate primer and/or the use of a soft face additive in the top-coats or clear lacquer to increase to increase the flexibility). (4 parts by vol paint or clear to 1 part additive before adding the hardener and thinner).

HOW TO REPAIR

Stone chips should be touched-up immediately. Refinish using the recommended undercoats and/or topcoats.


40.Uneven Color, Color Variation

DEFINITION

Uneven color In part of the paint finish with streaks or patches of lighter or darker tint.

CAUSES

[1]Poor dispersion of paint pigments; insufficient mixing of different color paint; use thinner with poor solubility; improper viscosity.

[2]Uneven spray thickness. In thickly sprayed parts, pigments convect and make color floating.

[3]Poor spraying technique: improper overlapping, too near from the paint surface, wrong angle between the spray gun and the surface (main reason).

[4]In the application site, there is gas source able to react with paint finish, such as ammonia, or carbon dioxide.

[5]Spray tools not clean enough.

[6]Incorrect spray nozzle and spray pressure.

[7]Unsuitable thinner.

HOW TO REPAIR

Allow the paint finish dry thoroughly, sand wet, then re-spray.