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newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Refinish: Surface Preparation, Mixing, and Paint Application › Lesson 6
TRN-AUTO25-05 · Lesson 6 · 16 clock hours · demo

Spray Gun Fundamentals: Setup, Adjustment, and Cleaning

Objective. Students will set up, adjust, and clean a conventional, an HVLP, and an LVLP spray gun, demonstrating correct fluid tip size selection, air-cap atomization pressure, fan width, and fluid delivery for primer and basecoat materials.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Spray Gun Fundamentals: Setup, Adjustment, and Cleaning

Why This Skill Matters

A spray gun is the most precise tool in the refinish shop. How you set it up and maintain it directly controls film thickness, atomization quality, and the number of defects that end up in the finished paint job. Employers and insurers expect technicians to work efficiently with HVLP, LVLP, and conventional guns, and both I-CAR REF07 and ASE B-series content cover gun operation and defect diagnosis. Master the fundamentals here and you reduce rework—the single biggest cost in a paint booth.

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The Three Gun Types

Conventional (high-pressure) guns operate at high atomizing air pressure at the air cap—typically 40–90 PSI—and produce excellent atomization but generate significant overspray. Regulations in many states restrict or prohibit conventional guns for topcoats because of VOC transfer-efficiency requirements. They remain useful for some primers and specialty materials.

HVLP (High Volume, Low Pressure) guns deliver a high volume of air but reduce air-cap atomizing pressure to 10 PSI or less. Federal and many state regulations recognize HVLP as compliant with transfer-efficiency standards (generally 65% or better). Gravity-feed HVLP guns are the most common gun in modern production shops for basecoat and clearcoat.

LVLP (Low Volume, Low Pressure) guns use less air volume than HVLP while still keeping air-cap pressure low. They are well suited to smaller compressor systems and also meet transfer-efficiency requirements. Atomization quality is comparable to HVLP when set up correctly.

Know which type your shop's air supply can support before you pick up a gun.

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Key Components and Their Functions

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Setup Sequence

1. Confirm the fluid tip size matches the material TDS.

2. Install the correct air cap for the gun type.

3. Attach an in-line pressure gauge at the gun inlet.

4. Set the fan knob and fluid knob to fully open, then back each off 1.5–2 turns as a starting point.

5. Pull the trigger and adjust the regulator to the inlet pressure specified by the gun manufacturer (commonly 26–29 PSI for HVLP gravity guns; always check the manual).

6. Spray a test pattern on masking paper held vertically at the recommended spray distance (typically 6–8 inches for HVLP).

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Reading and Correcting Test Patterns

Spray a 2–3 second burst, then examine the wet pattern shape:

| Pattern Defect | Likely Cause | Correction |

|---|---|---||

| Heavy center (football shape) | Air pressure too low or clogged horn holes | Increase inlet pressure; clean air cap |

| Heavy edges (donut/split sides) | Air pressure too high or fluid too low | Reduce air pressure; open fluid knob |

| Top/bottom split | Clogged or damaged horn holes | Clean or replace air cap |

| Narrow fan | Fan knob too far closed | Open fan knob |

| Spitting | Air in fluid passage or cup not sealed | Check cup gasket; check fluid tip torque |

Adjust one knob at a time and spray a fresh test pattern after each change. Record every adjustment and its result. This systematic approach is required in the activity and is the professional standard.

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Spray Technique Basics

Hold the gun perpendicular to the panel. Move at a consistent speed—typically 12–18 inches per second for basecoat. Overlap each pass 50% over the previous wet edge. Trigger the gun before the panel edge and release after, preventing buildup at the edges. Maintain a consistent distance; moving closer increases film build and increases the risk of sags.

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Cleaning Procedure

Proper cleaning protects the gun and prevents cross-contamination between materials—especially critical when switching from solventborne to waterborne products.

1. Empty remaining material from the cup back into its original container if still usable.

2. Add a small amount of manufacturer-recommended gun wash to the cup.

3. Spray the gun wash through the gun until fluid runs clear.

4. Disassemble: remove the air cap, fluid tip, and needle in the order specified by the manual.

5. Soak metal parts in gun wash; use the brush kit to clean fluid passages, the needle channel, and air-cap holes. Never use wire or metal picks on precision orifices.

6. Rinse with fresh gun wash, reassemble in reverse order, and torque the fluid tip to manufacturer specification.

7. Pour all used gun wash into the labeled solvent-waste container. Never pour down drains or into regular trash.

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Safety

Key terms10

HVLPHigh Volume, Low Pressure: a spray gun type that delivers large air volume at low atomizing pressure (10 PSI or less at the air cap), meeting regulatory transfer-efficiency requirements.
LVLPLow Volume, Low Pressure: a spray gun type that uses less air volume than HVLP while maintaining low atomizing pressure; suitable for smaller compressor systems.
Fluid tipThe precision-machined nozzle that controls the size of the fluid orifice; matched to material viscosity by orifice diameter (in millimeters).
Air capThe component mounted at the front of the gun that directs compressed air to atomize the fluid stream and shape the spray fan through center and horn orifices.
AtomizationThe process by which compressed air breaks the fluid stream into fine droplets for even application on a surface.
Dynamic (trigger-pulled) pressureThe actual inlet pressure measured at the gun with the trigger fully depressed and air flowing; the correct reading to use when setting up a spray gun.
Fan-width knobThe gun adjustment that controls air flow to the air-cap horn holes, increasing or decreasing the width of the spray fan ellipse.
Transfer efficiencyThe percentage of paint material that actually lands on and adheres to the target surface, as opposed to overspray; HVLP and LVLP guns are designed to meet regulatory minimums.
Technical data sheet (TDS)The manufacturer's document for a specific paint product specifying recommended fluid tip size, reduction ratios, spray pressures, pot life, and other application parameters.
Solvent-waste containerA labeled, sealed, chemically compatible container used to collect used gun wash and other contaminated solvents for regulated hazardous-waste disposal.

Worked examples4

1. Your HVLP gun manual specifies a dynamic inlet pressure of 29 PSI. You set the regulator with the trigger released and the gauge reads 29 PSI. When you pull the trigger and spray a test pattern, the pattern is narrow and the center is heavy. What is likely happening to the actual dynamic pressure, and how much might it be dropping?
  1. Recognize that the 29 PSI reading was taken statically (trigger not pulled). When air flows, pressure drops.
  2. Pull the trigger fully and read the in-line gauge while air is flowing. Suppose the gauge now reads 22 PSI—a drop of 7 PSI.
  3. A 7 PSI drop reduces atomizing air at the air cap, causing poor fan development and a heavy center pattern.
  4. Adjust the regulator upward while holding the trigger down until the gauge reads 29 PSI dynamically.
  5. Spray a new test pattern to confirm the fan widens and the heavy center corrects.
Answer: The actual working (dynamic) pressure was approximately 22 PSI—7 PSI below specification—because static pressure was set without air flowing. Re-setting to 29 PSI with the trigger pulled resolves the heavy-center, narrow-fan pattern.
2. A gravity-feed HVLP gun uses a 1.4 mm fluid tip for basecoat. The shop also has a 1.8 mm tip and a 2.2 mm tip. The technician needs to spray a high-build primer surfacer. Which tip should be selected, and why?
  1. Consult the primer surfacer TDS, which specifies a fluid tip in the range of 1.7–2.2 mm for that material.
  2. The 1.4 mm tip is designed for lower-viscosity materials like basecoat and clear; it will restrict flow and may cause tip dry or spitting with thick primer.
  3. The 1.8 mm tip falls within the TDS-recommended range and is the most controlled choice for a gravity-feed gun.
  4. The 2.2 mm tip is at the upper end; it could work but may deliver excessive fluid, increasing sag risk.
  5. Select the 1.8 mm tip as the best match for high-build primer surfacer.
Answer: Select the 1.8 mm fluid tip. It falls within the TDS-recommended range for high-build primer surfacer viscosity, providing adequate flow without the excessive material delivery risk of the 2.2 mm tip.
3. You spray a test pattern and measure the fan width at the top, middle, and bottom of the ellipse. The readings are: top 8 inches, middle 10 inches, bottom 8 inches. Does this pattern meet the lesson completion standard of within 10% width uniformity?
  1. Identify the maximum fan width: 10 inches (middle).
  2. Calculate 10% of the maximum width: 10 × 0.10 = 1 inch.
  3. Determine the allowable minimum: 10 − 1 = 9 inches.
  4. Compare each measurement to the allowable minimum: top = 8 inches, bottom = 8 inches—both are below 9 inches.
  5. The pattern fails the 10% uniformity standard because the top and bottom are 2 inches narrower than the center, a 20% deviation.
Answer: The pattern fails. A 10% tolerance on a 10-inch maximum allows no measurement below 9 inches. The top and bottom at 8 inches represent a 20% deviation. Likely cause is a clogged or damaged horn hole producing a top/bottom split; clean or replace the air cap and retest.
4. At the end of a spray session you have approximately 4 ounces of unused waterborne basecoat in the gun cup and 16 ounces of approved gun wash available. Outline the minimum volumes of gun wash needed for a two-flush cleaning procedure.
  1. Return the 4 ounces of unused basecoat to its sealed container to avoid waste.
  2. First flush: add approximately 4–6 ounces of gun wash to the cup, spray through until fluid runs mostly clear. Collect used wash in the waste container.
  3. Second flush: add another 4–6 ounces of fresh gun wash, spray through until the fluid runs completely clear with no color tint.
  4. Total gun wash used: 8–12 ounces for the two-flush procedure, well within the 16 ounces available.
  5. Pour all used gun wash into the labeled solvent-waste container immediately.
Answer: A two-flush procedure requires approximately 8–12 ounces of gun wash. With 16 ounces available, the technician has sufficient supply. All collected waste solvent must go into the labeled hazardous-waste container—never down a drain.

The math in this lesson1

Learners read static and dynamic air-pressure gauges, find a 7 PSI drop, and reset to 29 PSI at the gun.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Disassemble and identify every component of an HVLP gravity-feed gun; reassemble in correct order with instructor observing; torque fluid tip to manufacturer specification and confirm no air or fluid leaks at 29 PSI inlet pressure. · Spray test patterns on masking paper, adjusting fan, fluid, and air knobs one at a time to correct a heavy center, heavy edges, and top/bottom split pattern defect shown on a reference chart; record each adjustment and result. · After a simulated spray session using waterborne basecoat, disassemble the gun, flush with manufacturer-recommended gun wash, clean all passages with a brush kit, reassemble, and demonstrate correct solvent-waste disposal into the labeled container.
Materials: HVLP, LVLP, and conventional spray guns; fluid tip and air-cap sets; in-line pressure gauge; masking paper; gun cleaning kit; approved gun wash; labeled solvent-waste container; manufacturer gun manuals; I-CAR REF07 reference
Check: Test pattern on masking paper shows an elliptical fan within 10% width uniformity and consistent wet edge; gun disassembly/reassembly completed in correct sequence with no leaks at operating pressure; cleaning procedure observed as pass/fail with zero contaminated waste disposal errors.

Practice check7

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASE B2 – Surface Preparation and Refinishing
1. When verifying inlet pressure on an HVLP spray gun, the gauge reading should be taken with the trigger:
ASE B2 – Surface Preparation and Refinishing
2. A technician switches from a 1.4 mm fluid tip to a 2.2 mm fluid tip without changing any other settings while applying waterborne basecoat. What is the MOST likely result?
I-CAR REF07 – Safety and Environmental Compliance
3. Which personal protective equipment is required when spraying solventborne primer in a spray booth?
I-CAR REF07 – Safety and Environmental Compliance
4. After cleaning a spray gun with gun wash, the used solvent must be:
ASE B2 – Surface Preparation and Refinishing
5. A top/bottom split pattern—where material concentrates at the top and bottom of the fan with a thin band in the middle—most commonly indicates:
I-CAR REF07 – Spray Equipment Operation
6. Which of the following correctly describes the purpose of HVLP technology compared to conventional high-pressure guns?
I-CAR REF07 – Spray Equipment Operation
7. A technician is reassembling an HVLP gun after cleaning and must torque the fluid tip. The BEST source for the correct torque specification is:
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