The surface treatment before painting is a key step to guarantee the quality and durability of the paint. The life of the coating is affected by three main factors: the surface treatment (60%), the coating construction (25%) and the quality of the paint itself (15%). This highlights the importance of surface treatment. Different materials require different treatment methods. This article will focus on the surface treatment of metal and wood.
1. Metal surface treatment
Degreasing: The oil film on the metal surface needs to be removed. This can usually be achieved by using organic solvents, lye or electrochemical degreasing methods.
Rust removal: Oxide and rust residue on metal surfaces need to be removed. Methods include manual rust removal (e.g. sanding with sandpaper), mechanical rust removal and sandblasting. Sandblasting is the most effective of them all because it creates a good roughness on the metal surface, which helps improve the adhesion of the paint. Acid soaking is also a way to remove rust.
Paint Removal: If there is old paint residue on the metal surface, it needs to be removed. Methods include flame method, lye method and paint remover method. Paint remover mainly uses various organic solvents to dissolve or expand old paint. Once the paint softens, it can be easily scraped off.
Phosphating treatment: This is a process in which a workpiece (such as steel or aluminum, zinc parts) is immersed in a phosphating solution. The purpose is to form a water-insoluble crystalline phosphate film on its surface. This phosphate conversion film protects the base metal from corrosion and provides a high-quality substrate before coating. According to the temperature and time of treatment, phosphating can be divided into high temperature type, medium temperature type, low temperature type and normal temperature type. Each type has its unique advantages and disadvantages. Common methods of phosphating are chemical phosphating method, electrochemical phosphating and spray phosphating.
Passivation treatment: This is a process of sealing the pores of a phosphate film or forming a passivation film on the surface of a metal, designed to isolate the metal from the external medium. The commonly used method is chromate treatment.
2. Wood surface treatment
Before painting, the moisture content of the wood should be controlled between 7% and 12% to ensure that the paint will not crack, blister or fall off. Any stains or impurities on the wood should be removed with sandpaper. For better aesthetics, the wood also needs to be bleached and dyed before painting.
In conclusion, whether it is metal or wood, proper surface treatment is key, which can greatly improve the quality, adhesion and durability of the coating. Before painting, the appropriate surface treatment method should be selected according to the nature and characteristics of the selected material.
