ITI Mechanic Auto Body Repair (AITT Theory) cheat sheet

Fire classes: A = solids (trim foam, paper), B = flammable liquids (paint, thinner), C = flammable gases, D = combustible metals, E = electrical, F = cooking oils. Water must never be used on class B, E or F fires.

Isocyanate hardeners in 2K paint and clearcoat can cause occupational asthma. Spraying requires an air-fed full-face respirator plus booth extraction; a paper dust mask gives no protection.

The Safety Data Sheet has 16 standard sections: Section 2 = hazard identification, Section 4 = first aid, Section 8 = exposure controls and PPE, Section 13 = disposal.

Hearing protection action level is 85 dB(A) over an 8-hour day; because the scale is logarithmic, every 3 dB increase halves the permitted exposure time.

Rags soaked with paint, thinner or oil can self-heat and ignite. Store them in a metal bin with a self-closing lid and empty it daily.

Compressed air for cleaning must not exceed 30 psi (207 kPa) at the nozzle, and requires eye and ear protection plus a chip guard. Never direct air at skin.

Before lifting, place the hoist pads at the OEM lift points; support the vehicle on stands rated above its mass and chock the wheels. A hydraulic jack alone is not a support.

Welding galvanized steel releases zinc oxide fume that causes metal fume fever: grind the coating back 25 mm each side and use forced ventilation or an air-fed helmet.

Lockout/tagout: isolate and tag the main electrical breaker and the compressed-air supply before servicing a booth, hoist, compressor or other powered equipment.

Bumping hammer (cross-peen or curved face) raises low areas; the flat-faced planishing hammer finishes metal on-dolly; the serrated shrinking hammer gathers stretched metal.

Dolly selection: heel dolly for tight concave curves, toe dolly for convex crowns, general curved dolly for large panels. Hold the dolly behind the panel, opposite the strike.

A body file is drawn dry across a repair to reveal high spots; clean the teeth with a file card, never with the hand or by tapping the file on the bench.

Aviation snips: green = right cut, red = left cut, yellow = straight cut. Use them on sheet up to about 1.2 mm and never on wire, rod or double thickness.

Hacksaw blade choice: 18 TPI for thick solid section, 24 TPI general work, 32 TPI for thin tube. Keep at least three teeth in contact with the workpiece.

A cold chisel for mild steel is ground to a 60–70° included angle, for aluminium 30–40°. Dress a mushroomed head before further use or take the chisel out of service.

Use a torque wrench in the middle 60% of its scale, never to loosen fasteners, and recalibrate it at least annually; store it at the lowest scale setting.

A slide hammer with screw or weld-on tab pulls a dent outward; a stud welder deposits a small weld nugget for the puller and keeps heat input to the panel low.

Cracked, taped or loose hammer handles and chipped chisel edges must be removed from service. Regrinding overheated steel softens the temper.

Vernier caliper least count = one main scale division minus one vernier division; typical metric caliper 0.02 mm, outside micrometer 0.01 mm, dial gauge 0.01 mm.

Metric micrometer reading = barrel (sleeve) reading + thimble division × 0.01 mm. Check zero before use and always use the ratchet stop for consistent measuring pressure.

Feeler gauges measure panel gaps, plug gaps and clearance; combine leaves to build a gap and never force a blade, because a bent blade gives a false reading.

A tram gauge measures point-to-point distances such as door-aperture diagonals; equal diagonals on both sides indicate a square, undistorted opening.

Datum and control points are OEM reference holes listed on the body dimension chart. All underbody measurements are taken from these, not from panel edges.

Typical passenger-car panel gap is 4–6 mm with flush within about ±1 mm; check with a gap and flush gauge rather than by eye.

A prick punch (about 60°) marks layout lines; a centre punch (about 90°) locates a drill. Mark only on layout dye, never scribe a finished painted surface.

A combination square gives 90° and 45°; a bevel protractor reads angles; dividers and trammel points transfer arcs and radii from a pattern to the sheet.

Read a steel rule at eye level to avoid parallax error, and never use a measuring tool as a scraper or pry bar - a worn end loses about 0.5 mm of accuracy.

Sheet gauge: a higher number is thinner sheet. 20 SWG ≈ 0.91 mm and 18 SWG ≈ 1.22 mm; typical outer body panels are 0.6–0.9 mm mild steel.

Bend allowance BA = (π/180) × bend angle × (R + K·T), with K ≈ 0.33 when R < T and ≈ 0.5 when R > 2T. Setback = R + T; bend deduction = 2 × setback − BA.

Minimum bend radius is about 1 × T for mild steel and about 1–2 × T across the grain for 5052 aluminium (2–4 × T with the grain).

Run bend lines perpendicular to the rolling (grain) direction. If a bend must run along the grain, increase the radius to prevent cracking.

Springback rises with higher-strength steel and larger R/T ratio: overbend by roughly 1–3° for mild steel and 5–10° for high-strength low-alloy steel.

Annealing aluminium: heat to about 350–400 °C until a matchstick chars on contact, then quench or air-cool to restore ductility to work-hardened 5000-series sheet.

Shearing clearance is about 5–10% of sheet thickness per side; too little clearance causes secondary shear, edge cracking and a ragged cut.

Boron and high-strength low-alloy steels must not be heated above about 200 °C during straightening - heat destroys the temper and the designed crash performance.

A hemmed (folded) flange stiffens an edge and removes sharpness; the hem radius must be at least the sheet thickness or the fold will tear.

GMAW (MIG) on thin autobody steel: 0.6–0.8 mm ER70S-6 wire, 75% Ar/25% CO2 or 100% CO2, short-circuit transfer, 2–4 mm stitch welds with cooling between passes.

Oxy-acetylene neutral flame is about 3200 °C; the oxidizing flame suits brazing of brass, the carburizing flame suits aluminium and carbon removal.

Brazing filler melts above 450 °C and the base metal does not melt; soldering is below 450 °C. Brazed joints are not approved for structural panel replacement.

Resistance spot welding uses roughly 8–12 kA with squeeze, weld and hold times set by the controller; a peel test on a coupon confirms nugget diameter of about 4√t mm.

Distortion control: backstep (reverse) sequence, skip welding, copper backing bar, clamps or clecos spaced 25–50 mm, and cooling between short runs.

Remove zinc, paint, seam sealer and underbody coating 25–50 mm either side of the weld zone; zinc oxide fume causes metal fume fever.

Aluminium: AC TIG with high-frequency, or MIG with a push (forehand) technique, 4043 or 5356 filler, and dedicated stainless brushes and tools to avoid steel contamination.

Typical defects: porosity (gas or damp), undercut (current too high or travel too fast), burn-through (current too high or poor fit), lack of fusion (current too low).

Squeeze-type resistance spot welders reach joints a MIG torch cannot; apply weld-through primer between panels to restore corrosion protection at the joint.

Body-on-frame (ladder chassis) absorbs energy in the rails; a unibody uses its panels as the structure, so cutting a rail without OEM sectioning instructions compromises crash performance.

Five classic collision misalignments: mash (fore-aft), sidesway (lateral), sag (vertical drop), diamond (one side pushed back) and twist (opposite corners out of plane).

Measuring sequence: centreline and datum first, then diagonals, then panel gaps. Typical tolerance is ±3 mm on body dimensions and ±1 mm on mechanical mountings.

Straighten with several small pulls, relieving opposing damage first, and re-measure with a tram or three-dimensional system after each pull. Never pull on a two-post hoist.

Sectioning must follow the OEM procedure: specified cut location, backing insert, weld type (MIG butt, plug or squeeze spot) and full reinstatement of corrosion protection.

Crumple zones and reinforced members (B-pillar, rocker, rail) use high-strength steel; do not heat them and replace rather than repair where the OEM specifies.

SRS impact sensors mount on the radiator support, B-pillar and floor; their mounting points must be dimensionally correct or the system may mis-deploy.

Diamond damage is confirmed when one aperture diagonal is longer than the other; correct by pulling the long side forward and the short side rearward.

After structural repair, check wheel alignment and ADAS camera/radar aiming; a thrust-angle error points to a bent rear rail or axle.

On-dolly (planishing) traps the metal between hammer and dolly to stretch and flatten; off-dolly (bumping) holds the dolly off the panel behind the strike to raise a low area.

A high spot is stretched metal standing above the contour and is corrected by shrinking; a low spot sits below the contour and is corrected by bumping/stretching.

Heat shrinking: heat a 10–20 mm spot to dull cherry (about 600–700 °C), then quench with a wet rag or shrinking hammer while cooling, working from the outside inward.

Polyester body filler is catalysed with about 2% MEKP by weight and mixed on a clean board; maximum practical thickness is about 3 mm - deeper damage needs metalwork first.

Grit sequence for a filler repair: 40–80 to shape, 120–180 to refine, 240–320 to feather and prepare for primer; use a soft pad on the DA to avoid gouging.

Feather edge: taper the old paint edge over 25–50 mm so no step remains under the new finish; a sharp paint edge shows as a visible ring.

Paintless dent removal works only where the paint is intact and the metal has not been stretched past elastic recovery; it uses glue tabs, rods and a knockdown.

A stud welder with a copper-alloy stud and slide hammer pulls a dent without drilling; twist the stud off, grind flush and apply epoxy primer immediately.

Check crown (contour) by hand-feel across the panel and with a flexible profile gauge; the repair is finished when it matches the undamaged opposite side.

Thermoplastics (PP, PE, ABS, PC, TPO, PUR) soften with heat and can be welded; thermosets (polyester GRP, epoxy, SMC) char instead of melting and must be bonded or laminated.

Plastic welding: hot-air gun with a speed-weld tip at about 250–350 °C, filler rod of the same material as the panel, a V-groove along the crack and a 60–70° rod feed angle.

PP, PE and TPO have low surface energy and need flame treatment or a dedicated adhesion promoter before adhesive or paint will bond.

Two-part epoxy or polyurethane structural adhesive, mixed at its stated 1:1 or 2:1 ratio, is the approved repair for ABS, PC and PUR bumper covers, backed with reinforcement mesh.

Polyester resin for GRP is catalysed with MEKP at about 1–2% by weight; excess catalyst causes a hot, brittle cure that cracks and discolours.

GRP repair: grind a 12:1 bevel on each side of the damage, clean with acetone, apply gel coat, then build chopped-strand mat and resin in layers, finishing with sanding primer.

SMC (sheet moulding compound) needs SMC-specific repair paste or adhesive; ordinary polyester filler does not bond reliably to it.

Control exotherm by building thick laminations in layers of 3–4 mm with cure between them, or the resin can overheat, warp and burn.

Identify an unknown plastic first: sanding gives fine coloured paint dust versus larger plastic shavings, and a burn test on a hidden sample gives a characteristic smell (PP = candle wax, PVC = acid).

Refinish order: bare metal → epoxy primer → primer surfacer → sealer → basecoat → clearcoat. Each coat has a recoat window that must not be exceeded.

2K acrylic urethane and clearcoat contain isocyanate; use an air-fed full-face respirator in a supplied-air booth with full skin coverage - a charcoal mask is not adequate.

HVLP gun: about 2 bar (30 psi) inlet, roughly 10 psi at the cap; fluid tip 1.2–1.4 mm for primer, 1.2–1.3 mm for basecoat, 1.3–1.4 mm for clearcoat; spray distance 150–200 mm.

Overlap each pass by 50%, keep the gun perpendicular to the panel and trigger off at the end of the stroke to avoid heavy edges, runs and sags.

Flash-off between coats is typically 5–15 minutes at 20 °C, or until the film turns matte; recoating too soon traps solvent and causes solvent pop or blushing.

Compressed air must be dry and oil-free: water or oil in the line causes fisheyes and adhesion loss, so fit a coalescing filter and drain the tank daily.

Blending preparation: 600–800 grit on the repair, 1000–1500 grit to fade the edge, grey scuff paste on the rest of the panel; remove all silicone polish before painting.

Colour matching: test the variant against a spray-out card under booth lighting, and lay metallics wet-on-wet with uniform passes to keep the flake orientation consistent.

Typical dry film thickness: primer surfacer 40–60 µm, basecoat 15–25 µm, clearcoat 40–60 µm, confirmed with an ultrasonic or eddy-current DFT gauge.

Laminated glass is used for windscreens and cracks into a spider web held by the PVB interlayer; tempered glass is used for side and rear windows and shatters into small dice.

Bonded windscreen replacement uses one-part polyurethane adhesive with glass primer and activator, and the safe drive-away time (commonly 1–3 hours) must be observed.

The urethane bead should be triangular, roughly 8–12 mm wide and 12–15 mm high on the pinchweld; too thin a bead causes water leaks and weakens the bond.

Cut out with a cold knife, power cutter or wire; trim the old bead flush to the pinchweld without cutting into paint, and prime any exposed metal.

EPDM weatherstrip with flocked or sponge sections is fitted with soapy water or silicone-free lubricant, then checked with a hose test or a paper-pull test around the seal.

Seat upholstery is secured with hog rings, listing wires and J-clips; replace cut hog rings and warm vinyl gently with a heat gun to stretch out wrinkles without scorching.

Headliner adhesive must be rated for high cabin temperatures (about 100 °C or more); a low-temperature trim adhesive will sag in a hot cabin.

Forward-facing ADAS cameras behind the windscreen need static and/or dynamic recalibration after glass replacement, using the OEM target board at the specified distance.

Water leak diagnosis: run a hose test from the bottom of the suspected area upward while an assistant watches inside, and check sunroof drains and cowl clips.

Automotive systems are 12 V nominal with a negative-earth return; a parasitic draw above about 50 mA after the modules have slept indicates a fault.

Ohm's law: V = I × R and P = V × I. A 55 W lamp at 12 V draws about 4.6 A and needs a fuse of roughly 7.5 A (about 1.5 × running current).

A relay uses a low-current coil to switch a high-current circuit; a relay that clicks but passes no current has burnt contacts, and an open coil reads infinite resistance.

SRS: disconnect the battery earth and wait the OEM-specified time (commonly 1–3 minutes) before touching an airbag or its wiring; never probe an airbag connector with a multimeter.

CAN bus has two 120-ohm terminating resistors; measured across CAN-H and CAN-L with the bus asleep, the reading should be about 60 ohms.

Slow power windows usually indicate a dry or binding regulator or a weak motor rather than a faulty switch - check current draw and lubrication before replacing parts.

Heat-shrinkable crimp terminals are the accepted harness splice repair; solder joints embrittle and crack in a flexing harness, and twist-and-tape is not acceptable.

Corroded connectors cause resistance and intermittent faults: clean with contact cleaner, dry with air, apply dielectric grease, and replace the terminal rather than re-crimping a corroded wire.

LED or HID retrofits in a halogen housing spoil the beam pattern and are non-compliant; HID ballasts also require a high-voltage-rated harness and connector.

Estimate sequence: verify the vehicle and VIN, record the damage, look up the flat-rate labour time, price parts and materials, add refinish and sundries, then apply tax.

Labour is charged from the OEM or industry flat-rate time guide, not from clock time; refinish time is quoted per panel with allowances for blending and painting jambs.

Betterment is charged when the repair leaves the vehicle better than before the loss, for example a new panel on an older vehicle or a new tyre against a worn one.

A supplement (amendment) is raised when hidden damage appears after strip-down: photograph it, document it and obtain insurer or surveyor approval before proceeding.

Total loss or write-off: when repair cost plus salvage value exceeds the insured value (or a set percentage such as 75% of IDV), the insurer settles as a total loss instead of repairing.

The job card must record customer name, registration and VIN, odometer, fuel level, damage notes, accessories and a customer signature; it is the legal record behind the invoice.

Pre-repair and post-repair inspection sheets, a strip-and-inspect note, four-corner photographs and a delivery checklist protect both workshop and customer in a dispute.

Any variation to the authorised repair scope needs written customer approval - unauthorised work is generally not recoverable and can void the estimate.

State warranty terms on the invoice: the paint manufacturer's adhesion and colour warranty period plus a defined workmanship warranty, and retain the paint batch and mixing record.