How to solve the coating defects caused by different film defects?

1. Drawing and cobweb

When spraying, the paint atomizes poorly and sprays out in a silky shape, which makes the surface of the coating film appear as a silk net, which is called a wire-drawing spider web.

Etiology

1) The viscosity of the paint (related instrument: Viscosity Cup) is too high and the paint temperature is low.

2) The solvent power of the selected diluent is insufficient.

3) The resin content of easy drawing exceeds that of non-wire spraying.

prevention

1) Select the most suitable spraying air pressure, the most suitable construction degree, solid content and paint temperature for spraying.

2) Choose a diluent with appropriate (or stronger) dissolving power.

3) If the above two measures cannot solve the problem, the paint factory should adjust the paint formulation to reduce the content of easy-drawing resin, or use resin with uniform or lower relative molecular weight.

Remedy: To reduce the spraying pressure and % or viscosity, until the cobweb disappears.

Note: Some special materials, such as oil tank lining paint, are difficult to atomize and need special equipment to spray successfully.

Two, sweating

The phenomenon that one or several components are precipitated on the surface of the coating film. For example, when ordinary lacquer is dried above &'(, the plasticizer is precipitated in the form of beads of sweat, and the brilliance of the coating film reappears after being polished.

Etiology

1) The miscibility of plasticizer and paint base is poor. For example, nitro paint uses non-solvent plasticizers such as castor oil and camphor.

2) The coating film is not completely dry before sanding (the solvent is not completely evaporated).

3) When the coating film contains wax or mineral oil, it may gradually seep into the surface of the coating film.

prevention

1) Choose a plasticizer with good compatibility with the paint base, reduce the viscosity of the plasticizer and reduce the amount of non-solvent plasticizer.

2) The coating should be completely dry before sanding.

3) Edge shrinkage: During the coating and drying process, the coating film shrinks, which makes the coating film on the edges and corners of the object to be coated thinner, and in severe cases, the bottom may even appear. This phenomenon often occurs during the construction of water-based coatings. defect.

Etiology

1) The cohesion of the paint base is large.

2) The viscosity of the paint is low, and the solvent used evaporates slowly.

prevention

1) When designing the coating formula, attention should be paid to eliminating the disadvantages of edge shrinkage.

2) Add flow blocking agent to reduce cohesion.

3. Absorption

The paint is overabsorbed by the substrate during application and appears matte or unpainted. For example, when painting on fiberboard, the coating film can be seen just after painting, but it will disappear soon.

Etiology

The object to be coated is a porous material, such as pine boards, fiberboards, and scraped putty layers (loose), etc., and the paint coated on the surface is sucked into the pores, making the coating matte or incomplete.

prevention

1) The object to be coated with porous material should be coated with pore blocking paint and pre-treated (or surface adjusted) before painting.

2) For the surface that has been scratched by putty, primer or intermediate paint should be applied after grinding to eliminate the absorption of the topcoat by the putty layer.

3) Increase the number of coats, this measure is only applicable to occasions where the top coat is relatively cheap.

Four, sinking color, shadow

During the drying process of the topcoat, the topcoat loses its luster and evenly reflects the defects of the underlying coating film, such as the shape of putty repair blocks and substrate scratches. This phenomenon is called sinking. The phenomenon of uneven gloss due to poor sanding during repainting is called shadowing.

Etiology

1) Use coarse-grade sandpaper for poor sanding.

2) The base coat is dry-sprayed to form a penetrating hole.

3) The drying time between the layers is insufficient, resulting in inconspicuous bite of the primer layer.

4) The drying conditions are not good—narrow, cold, wet, and not ventilated.

5) The primer is sanded before it is completely dry.

6) The primer with high pigment content is not stirred every time before use.

7) Insufficient paint dilution and poor paint base dissolution.

(Improper trimming of the thin edge of the topcoat layer will also result in a dull appearance.

NOTE: Insufficient topcoat will result in a sunken appearance.

prevention

2) Avoid thick coating, and do not dry spray.

3) Primer should be stirred effectively before each use.

4) Seal all the putty, mud fill and sanded parts of the car body with primer.

5) Improve the drying conditions and increase the temperature of the painting workshop.

6) The thin edge of the joint for partial repair should be done effectively and correctly.

remedy

Allow the paint to dry effectively. This may take several days, or heat may be used to speed up its curing. Depending on the degree of color sinking, lightly use very fine sandpaper! Or coarse wax to smooth the surface, and then polish; in severe cases, use fine sandpaper to sand and re-spray.

Five, back sticky

The surface of the dried coating film appears viscous again.

Etiology

1) The paint used contains semi-drying oil.

2) Insufficient ventilation and high humidity after drying.

3) The alkaline substances contained in the substrate (such as cement wall) will saponify and soften the coating film.

4) The volatile matter of the primer layer gradually penetrates the topcoat layer to cause stickiness.

prevention

1) Change the type of paint.

2) Strengthen ventilation in dry places.

3) Before painting on the substrate containing alkali, it should be washed or coated with a sealing layer to prevent alkali.

4) The volatile matter of the primer coating should be completely volatilized before applying the topcoat.

Six, become brittle

The phenomenon that the elasticity of the coating film becomes poor, which is the prelude to cracking or peeling off of the coating film.

Etiology

1) Caused by over drying.

2) Coating matching is unreasonable, such as coating a high-temperature drying topcoat on a low-temperature drying primer layer.

3) The coating film with poor adhesion is easy to become brittle.

4) The coating film is too thick, or the ambient temperature is too low.

prevention

1) Select the appropriate drying specification through experiments, and select a coating with good compatibility.

2) Select the appropriate surface pretreatment method before painting to improve the adhesion of the coating film.

3) According to the conditions of use and the characteristics of the paint, select the appropriate film thickness (relevant instrument: wet Film Thickness Gauge) .

Seven, dissolve

The coating is damaged by the dissolution of aggressive liquid medium during use, accompanied by the phenomenon that the thickness of the coating film is thinned until the substrate is exposed.

Etiology

1) The paint used is not suitable for the use environment (poor resistance to certain media).

2) Contact with some corrosive liquid or gas during use.

prevention

1) According to the use conditions of the object to be coated, choose a coating with strong resistance to a certain erosive medium.

2) Prevent the coating from contacting with aggressive media and eliminate the source of erosion.

8. Moldy

During the use of the coating film, molds grow on the surface, causing the coating film to be destroyed.

Etiology

1) The environment of the object to be coated is humid, without sunlight or backlight.

2) The base material or substrate of the paint used may be the nourishment of mold (such as the base material of oily paint, wood, etc.).

3) The coating surface is not cleaned and maintained frequently during use.

prevention

1) Select mildew-resistant paint according to the use conditions of the object to be coated.

2) Add antifungal agents to the coatings used, and the substrates that are prone to mildew should be treated with antifungal treatment before painting.

3) The coating surface should be cleaned and maintained frequently.

Nine, dissolve

The phenomenon that the coated surface swells after being in contact with solvents, oils, adhesives, etc. during use.

Etiology

1) The coating used has poor stain resistance to solvents, oils, adhesives, etc.

2) The painted surface is not cleaned in time.

prevention

1) According to the use conditions of the object to be coated, select a coating with good stain resistance.

2) Frequently clean the surface of the object to be coated to eliminate contaminated foreign matter.

3) The coating film is too thick: the thickness of the dry coating film exceeds the upper limit film thickness specified in the process. When the appearance and performance of the coating film are still qualified, although it cannot be treated as a non-conforming product (that is, no coating film defect), it will cause the cost of coating to exceed the budget; and often cause other film defects.

Etiology

1) Abnormal film thickness due to repeated recoating.

2) Failure to operate according to the process specification, resulting in too thick coating film (defect of too thick paint film produced by electrophoretic coating).

3) The operator is unskilled, and the local excessive overlapping spraying (heavy gun) causes the local surface coating film of the object to be coated to be too thick.

prevention

1) Improve the passing rate of one-time painting and reduce repairing and re-painting.

2) Carry out the spraying (or electrophoresis) process in strict accordance with the process specifications.

3) Strengthen training to improve the proficiency of operators.