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The Causes Of 12 Common Paint Film Defects And The Solutions

3. orange peeling 1

Table of Contents

⑴, Sagging

1. sagging

Phenomenon: When we apply the paint on a vertical surface, the wet paint film moves downward due to poor sagging resistance, improper application, too thick paint film, etc., forming uneven coatings of various shapes and lower edge thickness.

Reason:

1. The solvent evaporates slowly.

2. Apply too thickly.

3. The spraying distance is too close, and the spraying angle is improper.

4. The viscosity of the paint is too low.

5. The temperature is too low.

6. There is almost no ventilation, and the solvent vapour content in the surrounding air is high.

7. The paint contains a relatively large pigment (such as barium sulphate) and is poorly dispersed in the paint base.

8. It is also easy to produce flow marks when new paint is applied to the old paint film (especially on the glossy paint film).

Precaution:

1. Proper selection of solvent.

2. It is advisable to control the thickness of one coating of conventional coatings at 20-25 μm. We can use a “wet-on-wet” process for thermoset coatings or high-solids coatings to obtain thicker coatings.

3. Improve the proficiency of spraying operation, control the spraying distance to spray large workpieces 25-30cm, spray small pieces 15-20cm, and move parallel to the object surface.

4. Strictly control the construction viscosity of the paint. For example, the spraying viscosity of nitro paint is 18-26 seconds, and the baking paint is 20-30 seconds.

5. Ventilate properly and keep the temperature above 10°C.

6. Add an anti-sagging agent to prevent flow mark defects, which has a better effect.

7. Sand the old coating before applying new paint.

⑵, wrinkling

2,wrinkling

Phenomenon: The paint film presents regular wrinkles in small-wave ripples, which can reach part or all of the film’s thickness.

Reason:

1. Coatings made with a large amount of tung oil are prone to wrinkling.

2. The proportion of driers in the paint needs to be balanced, and there are too many cobalt driers.

3. Sudden high-temperature heating and drying, the baking temperature of self-drying paint are too high.

4. The paint film is too thick, exceeding the norm.

5. Construction objects after dipping paint often have “thick edges” and are prone to wrinkling.

6. The volatile organic solvent is more likely to wrinkle than the slower volatile organic solvent coating.

Precaution:

1. When making paints containing tung oil, properly control the amount used.

2. Adjust the ratio of the drier and add other driers.

3. Increase the resin content in the coating components.

4. Strictly control the coating thickness.

5. Put the drying-type paint into the oven for 20-30 minutes, or add a zinc drier or an anti-wrinkle agent.

⑶, orange peel

3. orange peeling 1

Symptoms: The paint film presents a surface defect with an orange-peel appearance.

Reason:

1. The solvent evaporates too fast.

2. The levelling property of the coating itself is poor, and the viscosity of the coating is high.

3. Insufficient spraying pressure and poor atomization.

4. The spraying distance needs to be revised. If it is too far, the spray gun will run fast.

5. The wind speed in the painting room is too high.

6. The temperature is too high, enter the oven for drying too early.

7. The temperature of the object to be coated is high.

8. Insufficient coating thickness.

9. The surface to be coated could be smoother, affecting the paint’s levelling or absorption of the colour.

10. The moisture content in the nitrocellulose paint is related.

Precaution:

1. Select an appropriate solvent, or add some organic solvents with high boiling points that evaporate slowly.

2. Select a lower construction viscosity through experiments.

3. The compressed air pressure for spraying is 3.5 kg/cm2 for the suction-type spray gun with tank and 3.5-5 kg/cm2 for the gravity pressure feed-type spray gun to achieve good atomization.

4. Choose an appropriate spraying distance to reduce the wind speed in the spraying room.

5. Before the object to be coated enters the high-temperature drying furnace after spraying, there should be a drying room.

6. The temperature of the object to be coated should be cooled to below 50°C, and that of the paint and the painting room should be maintained at about 20°C.

7. The thickness of one layer of spraying should be within 20-26μm.

8. The surface to be coated should be fully wet-polished to make it smooth.

9. Add an appropriate levelling agent.

Coatings are prone to the above phenomenon: nitro paint, alkyd paint, amino paint, acrylic paint, and powder coating.

⑷, loss of gloss

4, loss of gloss 2

Phenomenon: The gloss of the paint film is reduced due to the influence of climate and environment.

Reason:

1. The quality of the solvent in the paint is poor, there are many low boiling point solvents, and the volatilization is too fast.

2. Too much thinner and drier will cause the surface of the paint film to dry too quickly and cause a loss of gloss.

3. Mixing of different types of paint, etc.

4. The construction viscosity is too high.

5. If the humidity is too high, greater than 80%, the volatile paint absorbs water, turns white, and loses gloss.

6. The bottom layer of construction is uneven and smooth.

7. Environmental impact.

Preventive measures:

1. Add some varnish to improve the gloss.

2. Modulate with the same type of paint.

3. Master and explore a set of construction viscosity.

⑸, whitish

5. foggy white

Phenomenon: During the drying process of glossy paint, the paint film sometimes appears milky white. Periodically it will return to its original appearance after drying.

Reason:

1. The air humidity is too high, above 80%, exceeding the proportion of water in the solvent, or the surface temperature of the coating film drops due to solvent volatilization during the drying process so that the local air temperature on the surface drops below the “dew point.” At this time, the air in the moisture condenses and penetrates the coating to produce emulsification, and the surface becomes a translucent white film. When the water finally evaporates, the voids are replaced by air to form a layer of porous and dull coating, thus reducing the decorative and mechanical properties of the paint film.

2. All organic solvents have low boiling points and fast volatilization.

3. The surface temperature of the object to be coated needs to be higher.

4. The object to be coated or the solvent used for dilution contains water.

5. Due to the failure of the oil-water separator used to purify compressed air during construction, water enters the paint and causes whitening.

6. The ratio of solvent and thinner needs to be revised. When part of the solvent evaporates rapidly, the remaining solvent cannot dissolve the resin, causing the resin to precipitate and turn white in the coating.

Precaution

1. Select organic solvents with a high boiling points and slow volatilization speed, such as butanol, butyl acetate, ethyl lactate and butyl lactate, diacetone alcohol, amyl acetate, cyclohexanone, etc.

2. Before painting, heat the object to be coated so that its temperature is 10C higher than the ambient temperature, or send the object to a warm oven to dry immediately after spraying.

3. The ambient temperature of the coating site is preferably 15-25°C, and the relative humidity should not exceed 70%.

4. The organic solvent and compressed air used should be checked for purity and no moisture.

5. Reasonable selection of organic solvents and diluents to prevent resin precipitation in the coating.

6. Add a moisture-proof agent.

Paints prone to whitishness: Nitro paint, perchloroethylene paint, phosphating primer and other volatile drying paints.

⑹, pinhole

6. pinhole

Phenomenon: A morbid state with pores similar to needle-punched in the paint film.

Reason:

1. Poor refining of varnish.

2. The choice of solvent and the mixing ratio need to be revised.

3. Poor dispersion of pigments.

4. The selection of additives and the mixing ratio are appropriate.

5. The surface tension of the paint is too high.

6. The fluidity of the coating is poor, and the flattening property is insufficient.

7. The release of bubbles of the coating is poor.

8. If the storage temperature is too low, the miscibility and solubility of the paint base will deteriorate, the viscosity will increase, or it will precipitate locally, which will quickly cause particles and pinholes.

Painting

1. Water, oil or other impurities remain on the surface of the coated object, which needs to be cleaned carefully.

2. The solvent volatilizes quickly, and its added amount is more than other solvents.

3. The viscosity of the coating is high, and the solubility is poor.

4. Stir vigorously for a long time, mix air into the paint, and generate countless bubbles.

5. When spraying, the air pressure is too high, which destroys the solvent balance in the wet paint film.

6. The temperature of the wet paint film rises too fast, and the drying needs to be more.

7. The object to be coated is hot.

8. Wet paint film or dry paint film is too thick.

9. Improper construction, the putty layer could be smoother.

Coating environment

1. The air circulation is fast, the temperature is high, and the wet paint film dries prematurely (referring to the dry surface).

2. The temperature is high, the oil-water separation of the spraying equipment fails, and the air needs to be filtered.

Precaution:

An effective way to prevent pinholes on the dry paint film surface is to eliminate the above causes. In actual production, it is better to strictly control the construction viscosity, reduce the viscosity and use slower volatile solvents to dilute the paint.

Paints prone to pinholes include two-component polyurethane, epoxy paint, amino-modified acrylic resin paint, alkyd paint, unsaturated polyester asphalt paint, and paint with less pigment (such as black paint).

⑺, bleeding

7,bleeding

Phenomenon: The diffusion process of coloured substances from the underlying layer (substrate or paint film) into and through the upper paint film, thus causing the paint film to exhibit undesired colouring or discoloration.

Reason:

1. The organic pigment or asphalt resin of the base coat is dissolved by the solvent of the top coat so that the colour penetrates the top skin.

2. The substrate (such as wood) contains coloured substances.

3. The topcoat contains organic solvents with solid solubility, and the coat is applied before the undercoat is completely dry.

Precaution

1. After applying the anti-bleeding primer, apply the topcoat.

2. Add flake pigments (such as aluminum powder, cloud iron, etc.) in the middle or top coat.

3. Use a solvent with a fast volatilization speed and a small dissolving ability for the underlying paint film.

4. A similar light-coloured primer can be used.

5. After the primer is dry, respray the primer.

⑻, bubbling

8. bubbling 1

Phenomenon: air or solvent vapour or both bubbles formed during the coating process, which may disappear during the drying process of the paint film or may exist permanently.

Reason:

1. There are residual hydrophilic substances such as oil, sweat, fingerprints, saline and alkali, and polishing dust on the painted surface.

2. The surface to be cleaned needs to be cleaned with rust remover.

3. The non-drying oil penetrates the wet wood surface. Not only will it foam after painting, but it will even be lifted off.

4. The wall is damp, and it is urgent to paint the construction. After painting, the water will diffuse outwards, the paint film will be pushed up, and it can be torn off in severe cases.

5. Wooden products are wet. After the paint is painted, the water evaporates and forms the paint film when heated. The thicker the paint film, the more serious the foaming will be.

6. The dry paint film should be stored at a high temperature for a long time.

7. If the bottom layer is not dry, the putty layer is not completely dry, scrape off the putty again.

8. The wrinkle paint coating is too thick, most of the solvent does not volatilize, the temperature in the drying room is too high, and the temperature rises too quickly.

9. The rust removal of the object needs to be cleaned, and it has been baked at a high temperature.

10. The air compressor and pipeline contain moisture.

11. Cast aluminum parts and aluminum parts with side seams are impervious to oil removal.

Precaution:

According to the cause, take corresponding measures to solve it.

⑼, shrink (laughing)

Phenomenon: After coating, the paint shrinks and does not adhere evenly, the wet or dry paint film is not smooth, the painted surface is partially exposed, and some are not attached, like writing on oil paper and ink.

Reason:

1. The old coating is too smooth.

2. The coated surface has wax, kerosene, silicone oil, engine oil, etc.

3. Solvent volatilization does not match the baking temperature. The solvent drying paint has a too-low boiling point, too-slow volatilization or poor solubility.

4. There is water on the coated surface.

5. The temperature difference between the coating and the object to be coated is significant.

6. Water and oil are mixed into the compressed air.

7. The surface of the object to be coated is sweaty, or the cleaning cloth and gloves used in the operation have been polluted.

8. Improper proportion of additives used in coatings.

9. Improper choice of solvent.

10. The paint has poor fluidity and poor flattening.

Precaution:

Take corresponding measures according to the above reasons.

⑽, hue messy

10 hue messy

Phenomenon: In the paint film containing mixed pigments, due to the separation of stains, some spots and stripes are inconsistent with the overall colour, making the hue messy.

Reason:

1. Poor dispersion of pigment in paint.

2. Pigments with very different specific gravity are added to the paint, and additives are not used.

3. Two or more different oils are used for blending, or two kinds of paints are blended and stirred unevenly.

4. Insufficient solvency of diluent.

5. When painting, ammonia and sulphur dioxide sources can interact with pigments nearby.

6. When painting, after applying dark paint, the paintbrush is not cleaned and then painted with light colour.

Precaution:

According to the above situation, take corresponding measures.

⑾, sweating

Symptoms: Matte oil-based paints and enamels appear glossy again after sanding. The plasticizer is precipitated as sweat beads when the nitro paint is baked above 60°C.

Reason:

1. The paint film is not completely dry (or the solvent is not completely evaporated) before sanding.

2. Nitro paint uses non-solvent plasticizers such as castor oil and camphor.

3. When adding paint to the paint film or on the surface of nitro paint, wax, mineral oil, or lubricating grease may gradually seep out of the paint film surface.

4. For linseed oil or clear oil with less resin content, the paint film is easy to sweat, generally damp and dark, especially in poorly ventilated places prone to sweating.

Precaution:

1. After the coating is dry, polish it.

2. Use solvent-based plasticizers.

3. When using paint, consider the characteristics of the paint. Clear oil with good wettability is suitable for outdoor and sunny environments.

4. Before painting, carefully wipe the wax and oil stains on the old paint film with gasoline before construction.

⑿, detaching from the substrate

12 detaching from the substrate 1

Phenomenon: When the same or different paint is applied to the dry paint film, the dry film under it softens, lifts, or detaches from the substrate during the application or drying of the coating (typical appearance such as lifting wrinkle).

Reason:

1. Apply the next coat of paint before the coating is completely dry.

2. The topcoat solvent can swell the primer.

3. Apply too thickly.

Precaution:

1. The base coat should be completely dry before applying the top coat, or the top coat should be applied after the solvent evaporate

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