Elective Units: Specialised Refinishing, Equipment and Quality Control
What the elective units cover
Elective units extend the core into specialised work: spray gun setup and maintenance, spray booth operation and bake cycles, advanced colour matching and blending, special-effect and multi-layer finishes, plastic and aluminium refinishing, paint defect diagnosis and correction, and small business or customer-service skills such as quoting and quality sign-off.
Concepts you must know
- Gun setup: fluid tip/needle/air cap combinations, fan pattern, atomising pressure, and the relationship between viscosity, temperature and flow. A wider fan with lower pressure gives better transfer efficiency but risks mottling.
- Booth operation: filter condition, airflow balance, booth cleanliness, flash-off between coats, and bake schedules per the product data sheet. Baking too soon or too hot causes solvent popping and die-back.
- Colour matching: variant chips, spray-out cards, and the effect of application (wet vs. dry coats) on metallic orientation. Tri-coat and pearl finishes require a control coat and careful blend panels.
- Blending: fade the colour within a panel, keep blend zones away from sharp body lines, and clear coat the whole panel where required.
- Defect diagnosis: runs/sags (too wet, too slow thinner, gun too close), orange peel (atomisation, viscosity, technique), solvent pop (too much film build or inadequate flash-off), blushing (moisture), and poor adhesion (contamination or wrong primer).
- Plastics and aluminium: flexible additives for bumper covers, adhesion promoters, and dedicated aluminium preparation to break the oxide layer.
Worked example / rule of thumb
A freshly baked panel shows fine pinholes across the surface. Rule of thumb: pinholes and popping mean trapped solvent - reduce film build per coat, extend flash-off, and check booth temperature and gun distance rather than simply adding thinner.
Exam traps
Distinguish the cause from the correction; a question may ask what to inspect first. Do not confuse defect names, and remember that product data sheets override habit.