Guangzhou Jinwei Chemical Co., Ltd

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Paint Defects Manual ( III )
- May 03, 2018 -

Paint Defects Manual (3)



11.Dust Contamination

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DEFINITION

Usually small, irregular particles in the paint film caused by foreign matter (e.g. dust/dirt) which can occur in different sizes, shapes, types and patterns.

CAUSES

[1]Poor cleaning of substrate prior to painting. 

[2]Lint containing overalls or clothes. 

[3]Spray booth dust problem due to dirty filters causing incorrect air supply, extraction and pressure. 

[4]Intake of contaminated air from outside the spray booth (polishing residues, fine dust etc.).

HOW TO AVOID

[1]Thorough cleaning before application of new paints. 

[2]Use lint-free overalls and cloth. 

[3]Ensure regular maintenance and filter changes of the booth. 

[4]Ensure polishing/finishing areas are away from the paintshop and separated by filtered and ventilated systems.

HOW TO REPAIR

For minor faults, polish with a fine polishing paste and finish with a high gloss polish. For severe faults, sand out the problem and repaint.


12.Dull Finish


DEFINITION

Although apparently smooth and evenly applied, the surface lacks shine.

CAUSES

[1]The viscosity of paint is too low.

[2]Too much thinner is added. Or use of poor quality thinner.

[3]Poor hold out of primer, or the application of topcoat over primer which is not thoroughly dry.

[4]Application over a poor substrate.

[5]The solid content of the paint is too low.

HOW TO AVOID

[1]Use proper primer, and allow to dry thoroughly before applying the topcoat.

[2]Add proper thinner according to recommended ratio.

[3]Ensure that the paint is stirred thoroughly, apply under the correct conditions using proper spraying techniques.

[4]Prepare the substrate thoroughly.


13.Edge Marking

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DEFINITION

Swelling or sweated edges showing in the top-coat around feather-edge sanded old paintwork.

CAUSES

[1]Insufficient drying of filler and/or knifing stopper areas.

[2]Swelling layer in the old paint at a feather edge sanded area where the layer of filler meets the old paint or metal. Or the feathered edge was not fine enough.

[3]Swelling of works primer on new/spare parts.

[4]Wrong process used over TPA or NC old finishes.

[5]Sanded areas not isolated properly before base-coat and clear processing.

HOW TO AVOID

[1]Ensure drying times are observed. Infra Red drying reduces the chances of edge marking by drying the lower layers first.

[2]Before painting, carry out a solvent test with an Acrylic or NC Thinner to check for solvent sensitivity. When sanding, ensure that the feather edge is fine enough on the layer being sanded (P80/P150) on stopper/filler, P240 on primer filler. Do not apply knifing fillers over sensitive paintwork (apply to bare metal only and leave a metal edge). Apply spray filler in light coats to isolate. Use paint with mild solvent properties.

[4]Observe correct mixing ratios.

[5]Use appropriate finishing system.

[6]Isolate sanded base with light coats of spray filler.

HOW TO REPAIR

Sand affected areas smooth and flat. Respray with the correct primer and/or top-coat.


14.Faults in Painting Plastic - Flaking

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DEFINITION

Delamination of the total paint film from the plastic substrate.

CAUSES

Insufficient cleaning of the plastic substrate.

Insufficient sanding.

Unsuitable cleaner or degreaser used.

Plastic part not heated before painting - this applies mainly to PU foam parts and polyamide (PA).

Plastic primer or plastic additive not used.

HOW TO AVOID

[1]Wash the part thoroughly with a mild detergent solution to remove any water soluble release agents. Thoroghly clean the plastic with appropriate cleaner to remove solvent soluble release agents. Where plastic is very dirty or textured, clean with the assistance of a sanding pad. Clean the surface before and after sanding.

[2]Use recommended sanding grades. Where an irregular shaped part is to be sanded(e.g. radiator grill, bumper etc.) use a sanding pad.

[3]Re-clean with appropriate cleaner.

[4]”Tempering” means sweating out the release agents by heating(1-2 hours at a maximum)

[5]Use appropriate plastic primer.

HOW TO REPAIR

Remove the flaking paintwork completely and refinish, Do not use a normal paint stripper or the plastic could be damaged or destroyed.


15.Faults in Painting Plastic - Cracking

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DEFINITION

Break or crack lines in the paint finish after mechanical stress. In severe cases the plastic may also crack. This problem is normally associated with soft flexible parts (e.g. PU rear spoilers).

CAUSES

[1]Insufficient or failure to use a flexible additive to the paint.

[2]Total paint thickness far to high.

HOW TO AVOID

[1]The amount of flexible additive added to the paint depends on the degree of flexibility required and whether the plastic is rigid (hard) or soft (flexible). Soft plastics are made of PU foam (e.g. rear spoilers) and can be easily indented with the thumb. All other plastics are rigid (hard). In principle, the filler, topcoat and clearcoat layers must be plasticised. Softface Additive is not used with basecoats.
Hard/rigid plastics: Mix the topcoat/clearcoat 4:1 by volume with additive before adding the hardener and reducer.
Soft/flexible plastics: Mix the topcoat/clearcoat 2:1 by volume with additive before adding the hardener and reducer.

Note:
First add the additive,
 and then add hardener in the recommended mixing ratio.

[2]Keep to recommended film thicknesses.

HOW TO REPAIR

If possible, remove the paint coats mechanically and then refinish. Do not use normal paint strippers as these could damage or destroy the plastic.