ITI Mechanic Auto Body Painting (AITT Theory) cheat sheet

PPE baseline for painting work: supplied-air or cartridge respirator rated for isocyanates (not a dust mask), nitrile gloves, splash goggles, coveralls, and hearing protection when using air tools.

Isocyanate hardeners in 2K clears and primers sensitise the airways; once sensitised, exposure to even very low concentrations can trigger asthma, so all mixing and spraying must occur with ventilation and respiratory protection.

Compressed air for breathing must be Grade D breathable air: dew point at or below 3 °C, no detectable oil mist, and CO monitored; a shop compressor feeding a paint gun is never a breathing source without an air-line filter and CO alarm.

Spontaneous combustion risk: rags and paper soaked with linseed oil, solvent-borne primer, or overspray residue must go into a self-closing metal bin or sealed water container, never a general waste bin.

Fire classification: Class A ordinary combustibles, Class B flammable liquids (solvents, thinners, paint), Class C energised electrical, Class D metals; solvent fires require foam, dry powder, or CO2, not water jet.

Flammable liquid storage: keep thinners and paints in approved cabinets, bond and earth metal containers when decanting, and keep ignition sources (grinders, heaters, welding) out of the spray zone.

Lockout/tagout applies to spray booth fans, compressors, and IR curing lamps before any maintenance; stored energy in air receivers must be bled to zero before servicing lines.

Housekeeping rule: keep walkways and booth floor free of overspray dust; wet-sweep or use a vacuum rated for combustible dust rather than dry sweeping, which aerosolises solvent-laden particulates.

Hand tool inspection: discard chisels and hammers with mushroomed heads or loose handles; air tool hoses need whip-check safety cables and couplers that retain pressure on disconnect.

Identify the substrate first: bare steel, galvanised steel, aluminium, plastic (PP, ABS, TPO, PUR), fibreglass/SMC, or existing OEM/e-coat finish each require a different abrasive and primer system.

Grit selection guide: 80–120 grit for stripping and heavy shaping, 180–240 grit for primer surfacer sanding, 320–400 grit for sealer prep, 500–800 grit for blending topcoat edges.

Galvanised steel must not be blasted or heavily ground; use a zinc-compatible etch primer or a wash primer, and remove the white oxide layer with a mild acid or dedicated cleaner.

Aluminium forms oxide within minutes of abrasion; apply an etch or epoxy primer within the recoat window stated on the TDS rather than leaving bare metal overnight.

Plastic identification: the burn test and float test distinguish PP/EPDM (needs adhesion promoter) from ABS/PUR; flexible parts require elastomeric additives in primer and topcoat.

Sanding direction and pattern: sand in a criss-cross pattern with progressively finer grits; guide coat (a mist of contrasting aerosol) reveals low spots and scratches before priming.

Surface cleanliness: wash with detergent and water, rinse, then wipe with a wax-and-grease remover using two clean cloths (one wet, one dry); silicone contamination causes fish-eyes that no sanding will remove.

Featheredge rule: blend the repair edge over a distance of at least 25–50 mm so the film thickness transitions smoothly; a sharp edge telegraphs through the topcoat.

Moisture check before coating: substrate and shop temperature must be above the dew point by at least 3 °C; otherwise condensation forms under the film and causes blistering or adhesion loss.

Mixing ratio is by volume, not weight, unless the TDS states otherwise: read the ratio as parts base : parts hardener : parts reducer, and use graduated mixing cups sized for the batch.

Pot life begins at the moment hardener is added; a 4:1 epoxy primer may have 8 hours at 20 °C but only 2–3 hours at 30 °C - heat roughly halves pot life for every 10 °C rise.

Reducer selection: fast reducer for spot repairs and cool conditions, medium for general use, slow for large panels or hot booths; wrong reducer causes dry spray or solvent popping.

Metallic and pearl orientation: apply a uniform wet coat at consistent gun distance and speed, then a drop coat at lower pressure to align flakes; uneven application changes the face colour.

Colour verification: spray a test card, flash off, and compare under three light sources (daylight, booth light, and a colour-corrected lamp) before painting the vehicle.

Variant selection: OEM colours have plant-to-plant variants; choose the variant chip matching the vehicle's build or spray out and tint rather than guessing from the code alone.

Tinting direction: add small amounts of the dominant toner to shift a colour; record each addition by weight so the formula can be repeated and entered into the mixing bank.

Catalysed products must be used within pot life and never returned to the original container; leftover catalysed material is waste, not stock.

Substrate compatibility: lacquer over enamel may lift; always check the TDS recoat window and test on an inconspicuous area when the previous finish is unknown.

Gun setup: fluid tip size matched to product - 1.2–1.3 mm for basecoat and clearcoat, 1.4–1.6 mm for primer surfacer and high-build; air cap pressure typically 2–2.5 bar at the gun inlet for HVLP.

Spray distance: 15–20 cm for conventional and HVLP guns; closer causes runs and mottling, farther causes dry spray and loss of gloss.

Overlap rule: 50 % overlap for uniform coverage; 75 % overlap when applying clearcoat or high-solids products to avoid striping.

Gun travel speed and trigger technique: start the pass off the panel, trigger on before the edge, release after leaving the edge, and keep the gun perpendicular to the surface through arcs.

HVLP versus conventional: HVLP transfers more material to the panel (higher transfer efficiency) at lower pressure but produces more overspray velocity issues; conventional atomises finer with higher pressure and more bounce-back.

Fluid adjustment test: a proper pattern is an even, oval fan with no split or heavy centre; a split pattern indicates a clogged cap or damaged needle/seat.

Booth parameters: downdraft booths typically run 0.2–0.5 m/s face velocity; a clogged filter drops velocity and raises overspray deposit and contamination.

Flash-off between coats: 5–15 minutes for basecoat depending on temperature and reducer; recoating too soon traps solvent and causes solvent pop in the clear.

Cleaning: flush the gun with the correct thinner immediately after use, disassemble the needle and cap for soak, and never leave catalysed clear in the gun past pot life.

Body filler is for cosmetic levelling over a repaired, sound substrate - maximum thickness typically 3–6 mm; deeper damage needs metal straightening or panel replacement first.

Surface prep before filler: grind to bare metal with 24–40 grit, remove all paint and rust, and leave a 50 mm featheredge around the repair area for the filler to key into.

Mixing filler: measure hardener at the ratio on the label (commonly 2 % by weight for polyester filler); under-catalysed filler stays soft and clogs paper, over-catalysed filler becomes brittle and porous.

Filler application: spread in thin passes with a flexible spreader, pressing material into the surface; a pinhole-free, slightly crowned shape sands flat more easily than a thick mound.

Sanding filler: shape with 40–80 grit on a firm block, refine with 120–180 grit, and finish with 240 grit; use a sanding block, not fingers, to avoid waves.

Dent removal sequence: push or pull the metal to near original contour first, then skim filler; pulling with a stud welder requires bare metal contact and a slide hammer with controlled force to avoid stretching.

Heat shrinking: excessive stretching from pulling causes oil-canning; apply a shrinking hammer or a controlled heat spot (dark red, then quench) to restore tension.

Plastic panel repair: identify the plastic, V-groove the crack, use a plastic welder or two-part plastic adhesive with a reinforcement mesh, and finish with a flexible filler.

Panel gap and alignment: check gaps and flushness with the adjacent panels before bodywork is finished; filler and paint build thickness that changes shut lines.

Primer types: etch primer for bare steel and aluminium, epoxy primer for corrosion protection and bare metal sealing, urethane primer surfacer for filling and sanding, and sealer for uniform topcoat holdout.

Etch primer must be applied thin (typically 10–20 µm) and never over body filler or existing paint; excess acid attacks the substrate and causes adhesion failure.

Epoxy primer recoat window: if the window is exceeded, the surface must be sanded or the next coat will not bond; check the TDS for the exact time at shop temperature.

Primer surfacer film build: 2–3 coats to 100–200 µm dry film, then block sand with 180–400 grit; guide coat each stage to find low spots.

Sealer is applied just before basecoat to lock the substrate, equalise absorption, and prevent solvent bleed-through from underlying layers.

Dry film thickness versus wet film: DFTh = WFT × (% volume solids ÷ 100); a 50 % solids product sprayed at 100 µm wet gives 50 µm dry.

Primer sanding defects: pigtails from trapped grit, sand-through to bare metal, and clogged paper from uncured primer; clean the panel and use a tack cloth before topcoat.

Recoating over primer requires full cure or the specified window; solvent trapped under a fast topcoat causes solvent pop and pinholing.

Basecoat application: 2–3 medium coats at 15–20 cm, flash off until matte; basecoat must never be sanded before clear unless the TDS permits it.

Clearcoat application: 2–3 coats with 50–75 % overlap, first coat a light tack coat, then a full wet coat; total DFT commonly 40–60 µm.

Solvent pop: caused by trapped solvent escaping through a partially cured clear; prevent with proper flash-off, correct reducer, and controlled film thickness.

Runs and sags: caused by too much material, too close a gun, or too slow a reducer; remove after cure by block sanding with 800–1200 grit and polishing.

Orange peel: insufficient atomisation, too high viscosity, or too fast a gun pass; correct with finer sanding and polishing or by adjusting gun setup.

Fish-eyes: silicone or oil contamination on the substrate or in the air line; clean with wax-and-grease remover and fit an oil/water separator on the air supply.

Blistering and lifting: moisture trapped under the film or a solvent attacking an incompatible undercoat; strip and rebuild with a compatible system.

Polishing sequence: 1000–1500 grit wet sand, then compound with a foam pad, then refine with a finishing polish; keep pad speed low to avoid burning the clear.

Curing: force-cure at the temperature in the TDS (commonly 60 °C for 30–45 minutes) and allow cool-down before polishing or reassembly.

VOC limits: many jurisdictions cap VOC content of automotive refinish products (for example, 420 g/L for clearcoat, 250 g/L for primer surfacer); check the local regulation and the product TDS before purchase.

Waste streams must be segregated: waste solvent, waste paint and catalysed material, empty metal containers, and contaminated rags each have separate disposal routes.

Waste solvent can be reclaimed by distillation on site or sent to a licensed contractor; never pour thinners down drains or into the ground.

Overspray filters and booth filter media are hazardous waste once loaded with paint solids; bag and label them for licensed disposal.

Air monitoring: isocyanate exposure limits are very low (often 0.005–0.02 ppm as an 8-hour TWA); use supplied-air respirators for spraying 2K products and fit-test respirators annually.

Environmental permits: spray booths and gun washers may require air discharge permits and trade-waste agreements; keep manifests and disposal records for the statutory period.

Noise and dust: air tools and sanding generate noise above 85 dB(A) and respirable dust; use water-based sanding or dust extraction to reduce airborne particulate.

Spill response: contain solvent spills with absorbent, avoid ignition sources, ventilate the area, and dispose of absorbent as hazardous waste.

Panel repair time is estimated in labour units; a common benchmark is 1.0 labour unit = 1 hour, with refinish time added per panel based on paint area and number of coats.

Paint quantity estimate: total area (m²) × dry film thickness (µm) ÷ (volume solids % × 10) gives litres of mixed paint needed; add 20–30 % for overspray and mixing loss.

Three-panel rule for blending: a repair on a panel usually requires blending into the two adjacent panels to match metallic colour, so the estimate must include them.

Measure paint area with a tape or panel area chart; include edges, jambs, and hidden returns that must be refinished for a complete repair.

Damage assessment: record panel condition, previous repairs, filler thickness, and corrosion on the estimate; hidden damage found after disassembly is handled as a supplement.

Colour match documentation: record the variant, spray-out card, and tint additions on the job sheet so the formula is repeatable for future repairs.

Customer handover: explain the cure time before washing or waxing (commonly 30 days for fresh clear), provide care instructions, and record the paint code and warranty terms.

Final quality check: inspect under proper lighting for dirt nibs, runs, colour match, panel gaps, and film defects; correct before delivery and document with photos.