Paint Composition, Types, Application, and Defect Analysis
Paintings
Definition: A fluid or fluidized material able to transform into a solid, opaque film, tenaciously adhered to the substrate on which it is applied, providing the color of the pigment in its composition.
All paintings share the characteristic of being applied in liquid form and becoming solid, continuous films once dried.
Components:
- Vehicle = solvent + binder
- Color Pigment
- Loads = additives
Component Properties: Resistance to the medium, adhesion to the substrate, chemical neutrality, color stability, performance.
Binder:
A liquid suspension that holds the pigments. It is responsible for forming the protective film; its performance and stability determine the final result. Raw materials include: Linseed oils, drying oils, alkyd resins, epoxy resins, polyurethane, and rubber.
Thinner (WT):
The component that provides fluidity to the mix, adjusting the paint’s viscosity.
Pigments:
Color Pigments: Insoluble materials in the vehicle or binder, whose purpose is to provide color and protect the paint. They also contribute to consistency, facilitating drying and opacity.
Metallic Dusts: Used for decorative purposes or to increase the corrosion resistance of some paints, including aluminum powder, stainless steel, nickel, or zinc.
Reinforcement Pigments: Inert pigments with weak tinting strength and opacity. Used for technical or cost reasons.
Fillers: Additives and pigments of low opacity, added to enhance properties, reduce the amount of pigment needed, and increase hiding power.
Types of Paint
Lime Paint: Lime slurry, with water as the vehicle.
Fresco: Coatings applied without drying, usually with lime.
Silicate Paint: Insoluble in water for 24 hours. The vehicle is potassium silicate.
Distemper (Tail): Made with animal fats, only used indoors. Water is the vehicle.
Oil Paint: Diluted with turpentine, made with vegetable oils.
Automotive Paint: The color is in suspension and improved with each hand-polishing using fine sandpaper.
Bituminous or Asphalt Paint: Obtained from a bitumen solution. Provides good protection for iron and performs well when buried.
Aluminum Powder Heat-Resistant Paint: Withstands temperatures up to 600°C, where the support is fixed.
Acid-Resistant Paint: Made with wood oils and synthetic resins.
Antioxidant Paint: Protects iron against oxidation, composed of red lead.
Light-Emitting Paint: Reflective: Contains small glass beads attached with adhesive. Phosphorescent: Exposed to light and glows in the dark. Fluorescent: Emits ultraviolet light.
Plastic Paint: Aqueous emulsion consisting of plastic, PVC, or polyurethane. Offers good covering power and durability; can be matte or satin.
Epoxy Paint: Synthetic resins formed by epoxy. Provides good covering power.
Paint can be applied in three ways: using a spray gun, a brush, or a roller.
Properties of Paints in Liquid State
Appearance: Visual inspection can provide information about color, skinning, homogeneity, floating, etc. Appearance is closely related to stability.
Viscosity: Indicates the presence of floating (separation of pigments or dyes), which can cause a lack of color uniformity in the dry paint.
Density: Determination of mass per unit volume. It is satisfactory when there is a correct pigment-to-vehicle ratio.
Particle Size: Pigments in paints should be well-dispersed, not agglomerated.
Flash Point: The minimum temperature at which vapors ignite a material in the presence of a flame.
General Recommendations for Application and Control
- Surfaces must be dry, and humidity and temperature should not lead to condensation but should allow for vapor release.
- Hot surfaces can cause paint boiling, leading to craters and blisters.
- Upon reception, check container labels. If the container is undamaged, stir the paint, let it stand for one hour, and apply within six hours; it becomes unusable after eight hours.
- Clean all equipment at the end of the day.
- Before painting, seal overlapping zones and contact areas between different materials with mastic.
- In corners and edges, apply a primer coat to prevent the paint layer from being too thin.
Paint Defects
Poor Brushability: Can be due to low temperature, improper brush, fast drying, or touching/rubbing the paint when the surface layer has begun to dry.
Slow Drying: May result in excessive film thickness, low ambient temperature, high relative humidity, excessive solvent use, and lack of ventilation.
Sagging: Results from a downward movement of the paint after application, before it dries, leading to areas of different thickness. It can be caused by excessive layering, over-dilution, and excessive solvent use.
Loss of Gloss: Caused by excessive paint dilution, moisture condensation, binder absorption when applied on a very porous substrate, use of improper solvent, or applying exterior paint on a not completely dry surface.
Lack of Adhesion: Caused by overly smooth surfaces, lack of primer, substrate with excessive moisture, grease or dirt on the surface, or flaking of old paint.
Poor Covering Power: Excessive contrast between the background and the paint, applying the paint too thinly, or lack of homogeneity in the paint.
Wrinkling: Formation of folds during drying. Caused by excessive layering, re-coating on a dry surface, or applying a layer with aggressive solvents that soften the underlying layer.
Cratering: Small depressions caused by poor wettability between layers. Caused by poor surface cleaning, contamination during painting, or incompatibility due to inadequate mixing.