Guangzhou Jinwei Chemical Co., Ltd

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Paint Defects Manual ( I )
- Apr 07, 2018 -

Paint Defects Manual (1)


1.Blistering

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DEFINITION

In damp weather, a small quantity of water vapour is absorbed into the paint structure and is then evaporated again in dry conditions (osmosis). This process is normal and does not harm a well constructed finishing process. However, poor processing of the primers will leave hygroscopic/water soluble substances (salts) behind as contaminants. These cause a local concentration of a salt water solution which lift the paint film into water blisters. Blisters can occur in many sizes, patterns and frequency and can form between individual layers or beneath the entire film build. In dry weather most blisters will temporarily recede.

CAUSES

[1]The surface to be painted (filler, bare metal etc) was not cleaned thoroughly. Contamination from salt residue, eg dirty sanding water or hand sweat, was left on the surface under or between coats. The blister pattern may indicate the cause (beading = wipe marks, prints = finger or hand prints). 

[2]Wet sanding operations (particularly with polyester based products) without sufficient time to allow water to evaporate before application of top-coats.

HOW TO AVOID

[1]Thoroughly clean the areas to be painted with clean water. Change sanding and cleaning water regularly, especially in winter when vehicle paintwork in for repair is covered in salt. Consider changing to dry sand operations to avoid water absorption when sanding. Finally (depending on the substrate) clean the area with an appropriate cleaner.

[2] Allow sufficient time for water to evaporate when wet sanding (circa 2 hours at 20ºC). Dry sand where possible especially with polyester based products.

[3]Ensure booth is dry before painting. In cold damp conditions run the booth on bake before any paint processing operations.

HOW TO REPAIR

Sand down and remove the paint layers back to a sound substrate. Be prepared to strip the areas back to bare metal where necessary. Refinish with the correct choice of primers fillers and top-coats.


2.Bleeding

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DEFINITION

Diffusion of a soluble dye from the substrate through a paint system. In most cases, bleeding leads to a spot-like discoloration of the topcoat, often producing a reddish or yellowish color shade. Excessive peroxide in a polyester body filler can also cause similar stains due to a chemical reaction with the pigments.

CAUSES

[1] Excessive peroxide from a polyester body filler causes a yellow-brownish stain in the topcoat. Blue and green colors are particularly vulnerable to this problem.

[2] Soluble dyes from the old paintwork are dissolved by the solvent of the repair materials and thus change the color of the surface.

[3] Residues from bitumen and tar.

HOW TO AVOID

[1] Carry out a solvent test to check if a soluble dye exists. Soluble dyes have not existed in any paintwork or repair paints for many years.

[2] Only use the recommended quantity of peroxide hardener when mixing polyester products and mix the hardener in evenly and thoroughly. Never allow the coloured hardener to look streaky in the filler when applied.

[3] Before painting ensure all contamination, especially bitumen and tar deposits, are throughly removed.

HOW TO REPAIR

To repair a bleed-through area, the whole area should be isolated with an appropriate sealer, and appropriate primer. If the bleed through is severe then the paint layers should be removed back to a sound substrate and then refinished with the correct primer and top-coat process.


3.Blushing

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DEFINITION

A milky gray cloud appears on the surface of the paint film.

CAUSE

When spraying during humid conditions, air from the spray gun and solvent evaporation lowers the substrate temperature below the dew point, causing moisture in the air to condense in or on the paint film. The condition is aggravated when

[1]Too fast drying or poor quality thinner is used;

[2]Fanning dry the paint film to speed up solvent release;

[3]Inadequate heating and / or air movement.

HOW TO AVOID

[1] Use good quality and proper thinner for sprayingconditions.

[2] Add the recommended amount of retarder solvent when spraying in humid conditions.

[3] Use proper gun set-up and techniques.

[4] Ensure the paint booth is properly ventilated, adequately heated.

HOW TO REPAIR

[1] Should blushing occur during application:

Apply heat to the affected area, or Add retarder solvent and apply additional coats.

[2] If the finish has dried, minor blushing may be corrected by compounding or polishing.

[3] Severe cases will require sanding and refinish.


4.Crinkling

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DEFINITION

Swelling and Lifting of the substrate layers when new paint is applied. Problem may appear during the painting process or on drying.

CAUSES

[1] Flash-off time too long when working wet-on-wet with some 2K-Products(recoating during Gel-phase).

[2] Recoating solvent-sensitive layers(NC/TPA) with the wrong repair materials or when layers are too thick.

[3] Coats used as a sealer are not thick enough(sprayed too thin or when sanded down too far).

[4] Substrate layers not thoroughly dried.

HOW TO AVOID

[1] Always keep to the recommended Flash-off times.

[2] Carry out a Solvent Test, use the correct repair process and use the correct film thickness.

[3] Use the correct Film Thickness as advised in the product technical data.

[4] Ensure that the substrate is dry.(Re-bake or use IR lamps)

HOW TO REPAIR

Sand down the paintwork back to a sound substrate. Rework using suitable Primer and Top Coats.

(If necessary be prepared to strip back to sound substrate).


5.Cratering (Fisheyes, Silicone Contamination)

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DEFINITION

Round recesses with a diameter from 0.5 to 3 mm. The appearance ranges from very flat depressions in the final layer of paintwork to serious wetting problems that go all the way down to the substrate. Craters that have been overcoated after inadequate refinishing can become visible again as flat recessions. 

CAUSES 

Mainly: Oil, grease, wax and silicon residue (e.g. polishes containing silicone): 

[1] Humans – impregnated working clothes, rubber gloves, skin and hair care products.

[2] Spraybooth – lubricants from moving parts, mould release agents from plastic hang-on parts, hoses and sealing compounds, poorly cleaned oil and waste water separators, dirty filters in ceiling and floor.

[3] Paint materials – incorrect use of additives (anti-silicone additive), unsuitable reducers (hardeners) from other suppliers, impurities in the coating due to deficiencies in transport and storage.

[4] Workpiece - residues from mould release agents on plastic parts, residual drawing agents and lubricants, soldering pastes, bitumen and oil from insulating mats. 

[5] Painting process additives - mould release agents from new sponge cloths, unsuitable cleaning agents and cloths, unsuitable sanding media (scouring cleaners), adhesive from adhesive tapes. 

[6] Environment – aspiration of contaminated air (polishing agent, silicone spray, fine dust, etc.) from other parts of the bodyshop, seals and insulating materials from the building. 

HOW TO AVOID 

Items 1-6 list possible sources of cratering. Consequently, various preventive measures to eliminate cratering can be established. As a basic principle, we recommend using only silicone-free products in paintshops and thoroughly cleaning the substrate.

HOW TO REPAIR 

Thoroughly sand the paint build back to a sound substrate. Repaint using the correct choice of primers and top-coats. If it is necessary to repaint over a paint layer where silicon induced craters are clearly present, we recommend the used of an anti-silicon additive to be applied.