Spray Finishing and Touch-Ups Summer 2024

Spray Finishing and Touch-Ups

• Overview of Spray Finishing: Equipment, materials, and techniques
• Two parts: Theory and Practical

Theory Topics

• Air Compressors
  • A supply of clean, dry air is required for many shop equipment
  • Type and size of compressor depend on selected spray equipment

Air Compressors

• A device that uses a motor/air pump to force air into a storage tank
• Compressed air is held in the tank until used
• When tank pressure reaches upper pressure limit, the motor shuts off
• When tank pressure drops, the motor turns back on

Compressed Air/Compressor Properties

• Pressure: Typically stated in pounds per square inch (psi)
• Delivery Rate (Flow Rate): Typically stated in cubic feet per minute (CFM) at a specific pressure
• Storage Volume: Typically stated in gallons or liters

Humidity

• Air can hold water vapor (moisture)
• Humidity: The amount of water vapor in the air
• Maximum amount of water vapor depends on the temperature and air pressure of the air
• Condensation occurs when air temperature drops

Compressors and Water

• When air is compressed, pressure increases, and water vapor condenses
• When air is released from the storage tank, air expands, and water temperature drops, forming condensation in air lines
• Compressor tanks and air lines collect water depending on the relative humidity

Types of Air Compressors

• Piston Type
  • Creates air pressure through the action of a reciprocating piston
  • Single Stage Piston
    • Air is drawn from the atmosphere and compressed to final pressure with a single stroke
    • Tank pressure typically kept between 100 psi and 125 psi
  • Two Stage Piston
    • Two cylinders: one large and one small
    • Air compressed in two separate steps
    • Higher pressure and more air can be stored in the same size tank
    • Advantage: More air can be stored; Disadvantage: Cost
• Rotary Screw Type
  • Utilizes two intermeshing helical rotors in a twin bore case
  • Air is compressed between one convex and one concave rotor
  • Trapped volume of air is decreased, and pressure increases
  • Quieter, higher CFM, and faster fill/recharge times

Making Liquid Flow

• Gravity
• Suction
• Pressurized Container (e.g., super soaker water gun)
• Pump

Spray Systems

• Types of Spray Systems
  • Conventional Spray Guns
    • Gravity Feed
      • Material is fed from a container above the gun via gravity
      • Liquid is atomized with compressed air
      • Pros: Well-suited for spraying both thin and thicker materials, relatively quick cleanup
      • Cons: Cumbersome in tight spaces, top-heavy, must be held upright
    • Siphon (Suction) Feed
      • Material is fed from a container below the gun via vacuum or suction
      • Liquid is atomized with compressed air
      • Pros: Well-suited for spraying thinner materials, relatively quick cleanup
      • Cons: Cumbersome in tight spaces, uses more compressed air, small capacity
  • Pressure Pot System
    • Gun + separate storage container for material
    • Liquid is atomized with compressed air
    • Pros: Gun is light, can be used in more confined areas, various container sizes
    • Cons: Care should be taken not to exceed the recommended pressure rating of the tank, long change over due to amount of cleaning needed
  • HVLP Turbine System
    • ‘HVLP’ – High Volume Low Pressure
    • Pressure is less than 10 psi
    • HVLP Gun + Electric Turbine
    • Turbine generates the required airflow
    • Air travels through a series of fans, each fan stage boosts the air pressure
    • HVLP Guns: Have larger passageways, available as siphon feed or gravity feed

Troubleshooting

• Blockage or debris on the fluid tip/needle
  • Rotate air cap 180 degrees to assess
  • Clean vent hole to resolve pulsating/fluttering issue

Respirators

• Can help protect against particulate hazards and chemical gases
• Selection of respirator type based on the type of hazard and level of exposure
• Respiratory Equipment Types
  • Negative Pressure
    • Relies on the wearer's lungs to pull air through the cartridge or filter
    • Potential to put strain on the wearer
    • Tight-fitting, requires fit testing and seal checks before use
  • Positive Pressure
    • Equipment pushes air to the hood or facepiece
    • Powered Air (battery-powered blower) or Supplied Air (bringing in clean air from outside the contaminated area)
    • May be loose-fitting or tight-fitting, facial hair compatible
  • SCBA (Self-Contained Breathing Apparatus)
    • User carries the source of clean air with them in a tank