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7 Practical Guidelines for Paint Workers
Article 3: Observing the Principles of Painting Systems
Key Points Note
As a rule of thumb, the undercoat is often designed not to fluidize, a technical measure to prevent cracking in multi-layered paint films.
Observing the Principles of Painting Systems
1. About the Choice of Painting Systems
The type of paint used for the undercoat determines the type of paint for the topcoat. Here, we represent two types of paints as Chocolate and Cookie. Their definitions are as follows:
Chocolate Type Paint: Dries solely by solvent evaporation, turning into a solid. The paint film can be redissolved, and the molecular weight of the resin does not change.
Cookie Type Paint: Resins react chemically with each other, increasing the molecular weight of the resin and forming a physically strong paint film. It does not flow even when heated.
For example, if Chocolate is chosen for the undercoat, Chocolate must also be chosen for the topcoat. Conversely, if Cookie is chosen for the undercoat, both Chocolate and Cookie can be chosen for the topcoat. However, the combination of a Cookie topcoat with a Chocolate undercoat should be avoided.
A fundamental principle of painting is to “use more pigments and fillers (solid particles) in the lower layers.” This design ensures that the undercoat remains immobile even when overcoated.
2. Defects in Overcoating
Defects due to overcoating include cracking and wrinkling.
Wrinkling: This is a ‘swelling phenomenon’ that occurs when the topcoat is applied while the undercoat of Cookie is in a semi-hardened state.
Cracking: Cracks occur during the drying process of the topcoat when its solvent penetrates the undercoat. At this time, the topcoat film undergoes volume shrinkage, and this shrinkage force acts as a pulling force. The condition for cracking is ‘shrinkage force > tensile strength’, but usually, the tensile strength of the paint film is greater than the shrinkage force, making cracking rare in the combination of Chocolate + Chocolate. Shrinkage force refers to the internal stress acting in various directions within the paint film, while tensile strength refers to the bonding strength or toughness of the paint film. The easier it is for the undercoat to fluidize, the more pronounced the cracking. If the undercoat is Chocolate, the following conditions will result in cracking, regardless of the type of topcoat:
The undercoat fluidizes with the solvent of the topcoat (does not fluidize if the undercoat is Cookie).
The shrinkage force of the topcoat film exceeds its tensile strength.