Spray Painting
Spray painting atomises liquid coating onto a prepared surface, building a film that cures into a decorative and protective layer.
- Part
- Finishing
- Revised
- 2026-08-11
At a glance
- Family
- Additive
- Typical tolerances
- A complete system adds 4–6 mils (100–150 µm) per coated surface; budget 2× on a bore diameter and roughly 4× on thread pitch diameter, or mask
- Surface finish
- Flat through full gloss; the film smooths but does not fill weld spatter, porosity or deep sanding scratches
- Typical volumes
- 1 part to continuous automotive lines; the most viable option at volume one
- Lead time
- 2–10 business days at a job shop depending on the number of coats and cure schedule; field work is scheduled by weather window
- Materials
- Metal, Plastic, Wood, Composite
What it is
Spray painting atomizes a liquid coating and delivers it to a prepared surface, where it flows out and cures into a decorative and protective film. It remains the most flexible finishing process available: it accepts any substrate that tolerates the cure, produces the full color and gloss range including automotive Class A finishes, can be applied in the field, and can be spot-repaired — none of which powder coating or e-coating can do.
Typical film builds are 1–2 mils (25–50 µm) for a primer, 0.5–1 mil (13–25 µm) for a basecoat, and 1.5–2.5 mils (38–63 µm) for a clearcoat, giving a complete system of roughly 4–6 mils (100–150 µm). Delivery method sets transfer efficiency and therefore both cost and emissions: conventional air spray transfers roughly 30–40% of what leaves the gun, HVLP (defined by an atomizing air pressure of 10 psi or less at the cap) reaches around 65%, and electrostatic bell application goes considerably higher.
How it works
- Clean and abrade. Solvent wipe to remove oil and release agents, then abrade — typically P320–P400 for a primer key. Any silicone contamination produces fisheyes that cannot be sanded out of a single coat.
- Pretreat. Bare steel needs a phosphate conversion coating or an etch primer; bare aluminum needs a chromate or non-chrome conversion coating or a self-etch primer. As with every coating system, this stage sets corrosion performance far more than the topcoat does.
- Prime and block. A build primer or primer-surfacer fills sanding scratches and minor substrate defects, then is block sanded flat. This step is what separates a Class A finish from an ordinary one.
- Spray the topcoat. Gun setup — fluid tip typically 1.3–1.4 mm for basecoat and 1.4–1.8 mm for primer — plus 6–8 in (150–200 mm) gun distance and 50% pass overlap produce a uniform film. Dry film thickness follows from wet film thickness multiplied by the coating's volume solids, which is the arithmetic used to plan how many coats are needed.
- Flash and cure. Solvent flashes between coats, then the film cures — air dry, force dry at roughly 140–180 °F (60–82 °C), or a full bake. Two-component polyurethanes and epoxies crosslink chemically and reach full hardness over days.
- Polish if required. Class A work is denibbed, compounded and polished after cure to remove dust inclusions and orange peel.
Delivery methods
Conventional air spray atomizes finely but wastes most of the material. HVLP and LVLP trade atomization quality for transfer efficiency and are the regulatory default. Airless pumps fluid at 1,000–3,000 psi through a small orifice with no atomizing air, delivering high film build fast on large structures. Air-assisted airless adds a small amount of shaping air for a better finish. Electrostatic charges the atomized droplets so they wrap around the part, giving the highest transfer efficiency on racked metal parts.
Environmental window
Substrate temperature should be at least 5 °F (3 °C) above the dew point, and most systems specify 65–80 °F (18–27 °C) with relative humidity below roughly 70–80%. Spraying outside that window causes blushing, solvent popping and adhesion failures.
Design guidelines
Break sharp edges
Liquid paint, like powder, pulls away from a sharp edge as it flows out and cures. An unbroken edge carries a fraction of the film thickness of the adjacent flat and is where corrosion and chipping start. Break outside edges to at least 0.020 in (0.5 mm), and more on parts that must survive a long salt-spray requirement.
Design for gun access
Spray is line-of-sight. Deep pockets, channel interiors, blind recesses and the back sides of closely spaced ribs receive little coating. Where the interior of a hollow section must be protected, use e-coating, a cavity wax, or design the part to be coated before assembly.
Provide drainage, venting and a hanging point
Cups and pockets hold solvent and washer solution, which causes runs and solvent popping. Add drain holes at the low point in the hang orientation, vent enclosed volumes, and provide a hanging feature so the part can be racked without the operator improvising one.
Mask everything with a fit or a ground path
At 4–6 mils for a full system, threads, bores, bearing seats, sealing faces and grounding pads must be masked or machined afterward. Thread pitch diameter grows by roughly four times the radial film thickness.
Specify the system, not the color
A drawing note that gives only a color leaves the corrosion performance undefined. State the pretreatment, primer, topcoat chemistry, dry film thickness range and the acceptance tests — adhesion by ASTM D3359 cross-hatch, thickness by ASTM D7091, and a salt spray requirement if one applies.
Set expectations for substrate telegraphing
Paint at 4–6 mils smooths but does not fill. Weld spatter, porosity, deep sanding scratches and blast profile all read through a gloss finish. Class A appearance requires block-sanded primer and a substrate prepared to match; see the surface finish chart for how prep methods compare.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Primer film thickness | 1–2 mils (25–50 µm) | — | Fills sanding scratches and provides adhesion |
| Topcoat film thickness | 1.5–2.5 mils (38–63 µm) | Excess film runs and sags | Liquid coatings have no self-limiting mechanism |
| Complete system | 4–6 mils (100–150 µm) | — | Typical primer plus base plus clear |
| Outside edge break | 0.020 in (0.5 mm) minimum | — | Coating thins at sharp edges |
| Thread allowance | ~4 × film on pitch diameter | Mask instead | Thread geometry multiplies radial build |
| Substrate vs. dew point | At least 5 °F (3 °C) above | — | Condensation causes blushing and adhesion failure |
Cost drivers
Spray painting is priced by labor and booth time. Material is a minor share, especially at low transfer efficiency where most of what is sprayed never reaches the part.
- Surface preparation. Sanding, filling, priming and block sanding dominate the cost of any appearance finish. The paint itself is the cheapest part of a Class A job.
- Number of coats and colors. Every additional coat is another booth cycle plus flash and cure time. Two-tone work adds masking between coats.
- Masking. Hand-applied, hand-removed, per part.
- Transfer efficiency. Conventional air spray wastes 60–70% of the material and generates the corresponding VOC and waste-disposal burden. Electrostatic and HVLP reduce both.
- Rework and inspection. Dust inclusions, runs and orange peel on a gloss finish are visible at a glance, so appearance work carries a high denib-and-polish and rework cost.
Cost-reduction tactics:
- Specify the gloss and appearance grade the part actually needs — a satin finish hides substrate defects that a high gloss will magnify and hides the cost of correcting them.
- Use HVLP or electrostatic application to cut material consumption and emissions.
- Design in a hanging point and drainage so parts rack quickly and do not need rework for runs.
- Reduce masked features by moving fitted surfaces onto separate components.
- At volume on metal parts with no heat-sensitive components, price powder coating instead — it is usually cheaper per part, needs no solvent, and is tougher, at the cost of appearance flexibility and repairability.
Questions
6 questionsHow thick is a sprayed paint system?
A typical complete system is 4–6 mils (100–150 µm): roughly 1–2 mils (25–50 µm) of primer, 0.5–1 mil (13–25 µm) of basecoat and 1.5–2.5 mils (38–63 µm) of clearcoat. Dry film thickness equals wet film thickness multiplied by the coating's volume solids, which is how coat counts are planned.
What is HVLP and why does transfer efficiency matter?
HVLP means high volume, low pressure — defined by an atomizing air pressure of 10 psi or less at the air cap. It transfers roughly 65% of the material to the part, against about 30–40% for conventional air spray. Higher transfer efficiency cuts material cost, VOC emissions and waste disposal, which is why it is the regulatory default.
Spray painting or powder coating?
Spray painting for heat-sensitive substrates, Class A automotive appearance, field application, spot repair and one-off work. Powder coating for a thicker, tougher film at lower cost per part in volume, with no solvent — but it requires a conductive part that survives a 350–400 °F (177–204 °C) bake and cannot be spot repaired.
What conditions does spray painting require?
Substrate temperature at least 5 °F (3 °C) above the dew point, with most systems specifying 65–80 °F (18–27 °C) and relative humidity below roughly 70–80%. Spraying outside that window causes blushing, solvent popping and adhesion failures that cannot be corrected without stripping.
Why does paint fail first at edges and corners?
Liquid coating pulls away from a sharp edge by surface tension as it flows out, so the edge ends up with a fraction of the film thickness on the adjacent flat. Break outside edges to at least 0.020 in (0.5 mm), and more where a long salt-spray life is required.
How do I specify a paint finish on a drawing?
Name the pretreatment, the primer, the topcoat chemistry, the dry film thickness range and the acceptance tests — adhesion by ASTM D3359, thickness by ASTM D7091, and a salt-spray requirement if one applies. A color alone leaves corrosion performance completely undefined.