Acetone (2-propanone, CAS 67-64-1) is assigned to the ketone solvent class and is selected for pre-paint cleaning where a rapid, non-residue-evaporating solvent is required. The solvent has a density of 0.790 g/cm³ at 20 °C, a normal boiling point of 56.1 °C, and a closed-cup flash point of -17.8 °C as measured by ASTM D56. The vapor pressure is 24.7 kPa at 20 °C, which gives an evaporation rate of approximately 5.6 relative to n-butyl acetate under ASTM D3539. The lower and upper explosive limits in air are 2.5 vol% and 12.8 vol%; the autoignition temperature is 465 °C. Acetone is miscible with water and most organic solvents, a property that supports removal of polar and nonpolar soils, but it also means that acetone cannot be used as a water-displacing solvent on wet surfaces. The purity and water content of technical grades are covered by ASTM D329-22, which is invoked for coatings-grade solvent because nonvolatile residue and acid content influence paint intercoat adhesion. When acetone is used before painting, it dissolves cutting oils, fingerprint oils, waxes, plasticizer residues, light greases, and some uncured resinous contaminants; it does not remove rust, mill scale, inorganic salts, or thick polymerized linings. Operators should not assume that solvent cleaning alone meets the full surface preparation requirements of an industrial painting specification.

How Does Acetone Remove Fabrication Oils from Mild Steel Before Epoxy Priming?

On mild steel substrates prepared for structural epoxy primers, the solvent cleaning step defined in SSPC-SP 1 is a preliminary operation that removes visible oil, grease, dirt, and drawing compounds. The recommended procedure uses a two-wipe method with polyester nonwoven wipes having a basis weight of 120–150 g/m² and a lint content below the threshold specified for cleanroom applications. The first wipe is saturated with acetone and applied in a single linear pass to dissolve the soil; the second dry wipe follows immediately to absorb the solvent-soil mixture before evaporative cooling allows re-deposition. Wipes are changed at intervals of 0.25–0.5 m² of cleaned surface because the soil-loading capacity of a wipe is finite and visual inspection alone is not a reliable indicator. Mechanical agitation with a scrub pad may be required for tenacious lanolin or paraffin films, but acetone alone is insufficient for polymerized or oxidized cutting fluids. After solvent cleaning, the surface is checked by water break per ASTM F22; a continuous water sheen that does not bead or separate for 30 seconds indicates the absence of hydrophobic residues. If the water film breaks, the solvent cleaning must be repeated with fresh wipes. This test does not remove mill scale or rust; abrasive blasting to ISO 8501-1 Sa 2½ or SSPC-SP 10 near-white metal may be required by the primer data sheet. Cleaned steel is susceptible to flash rusting when the relative humidity exceeds 60% or when the steel temperature is within 3 °C of the dew point; the interval between acetone cleaning and paint application should not exceed 4 hours unless a rust-tolerant primer is specified. Pull-off adhesion testing per ASTM D4541-22 is often specified at a minimum of 5 MPa for epoxy primer on blast-cleaned steel, but this value is system-dependent and must be taken from the coating manufacturer’s published technical data.

On aluminum alloys such as 6061-T6 and 2024-T3, acetone wiping is frequently inserted between aqueous alkaline cleaning and chemical conversion coating. Acetone removes machining coolants, temporary protective oils, and finger oils that can interfere with chromate or non-chromate conversion films. Unlike steel, aluminum develops an aluminum oxide/hydroxide layer that changes surface energy; water-break testing per ASTM F22 is useful but may give false positives if the surface is sanded or if absorbed moisture is present. Acetone does not etch or desmut the metal, so a conversion coating step such as MIL-DTL-5541F Type I or II, or a commercial trivalent chromium process, remains mandatory before painting. Where the specification requires a bare aluminum surface without conversion coating, coating adhesion is evaluated by cross-cut per ASTM D3359-23 or by pull-off per ASTM D4541-22, but the numeric acceptance level is dependent on the paint family. Acetone wiping of aluminum must be followed by a minimum evaporation period of 15 minutes at 20–25 °C because trapped solvent in lap joints or crevices can cause solvent popping during bake cycles above 80 °C. In production systems, forced air knives or high-velocity air nozzles operating at 550–700 kPa remove particulate residues after the solvent has evaporated.

Solvent Wipe Techniques on Epoxy-Based Thermoset Composites

For carbon fiber-reinforced epoxy laminates, release agents such as silicone, PTFE, or proprietary polymer films are a common barrier to paint adhesion. Acetone can dissolve some silicone oils and many solvent-borne release agents, but it does not remove all siloxane crosslinked films. The recommended solvent wipe procedure is to dampen a clean polyester knit wipe with acetone, apply it with a single-pass pattern that avoids flooding, and immediately dry the surface with a second dry wipe. The total liquid contact time should not exceed 30–60 seconds because the epoxy matrix can absorb solvent, causing reversible plasticization and a temporary reduction in glass transition temperature. Extended immersion is not permitted for structural laminates; published data from the composite manufacturer should be consulted for allowable solvent exposure limits. After solvent cleaning, the surface is often abraded with 180–240 grit silicon carbide paper or a fine abrasive pad to mechanically disrupt release agent layers, followed by a second dry wipe and tack rag. The solvent-cleaned surface can be checked by water break per ASTM F22, but this test does not verify siloxane contamination; FTIR analysis per ASTM E1252 or X-ray photoelectron spectroscopy on representative panels is used when adhesion failure risk is high. Primers intended for composite substrates are typically epoxy or isocyanate-based, and their adhesion on acetone-wiped laminates may be measured by pull-off per ASTM D4541-22 or by scratch adhesion per ASTM D7027. The operator must observe the primer manufacturer’s maximum recoat window because excessively dried or contaminated surfaces can fail before topcoat application.

Acetone is a strongly aggressive solvent toward many thermoplastic substrates used in painted components. Polycarbonate, acrylic, polystyrene, and acrylonitrile-butadiene-styrene can exhibit stress crazing, solvation, or whitening within minutes of exposure; compatibility evaluation per ASTM D543-20 should be carried out on molded plaques of the same thickness and residual stress state as the production part. Semi-crystalline polymers such as polypropylene and polyethylene are less visibly attacked by acetone, but degreasing with acetone does not increase the inherently low surface energy of these materials. They require flame, corona, or atmospheric plasma treatment to bring wetting tension above 38–40 mN/m as measured by ASTM D2578-17. If acetone is used on polyamide or polybutylene terephthalate, the contact time should be kept below 30 seconds, followed by a dry wipe and a minimum 30-minute flash-off period before priming; retained ketone solvent can interfere with isocyanate-hydroxy crosslinking in two-component polyurethane topcoats. Pinhole formation and loss of distinctness of image are common failure modes associated with retained solvent. On painted thermoplastic components, solvent wiping before repainting can lift edge mapping or partial films; mechanical adhesion testing per ASTM D3359-23 on test plaques should be used to establish that the wipe procedure does not reduce the rating below 4B.

When Acetone Is Applied to Galvanized Steel After Solvent Cleaning

Galvanized steel substrates present a separate set of constraints because the zinc layer is chemically active and may retain passivation treatments from the galvanizer. Acetone is effective in removing fresh mill oil and handling deposits, but it will not remove white rust, zinc oxide, or corroded storage stains. The painting of hot-dip galvanized steel is addressed by ASTM D6386-16, which distinguishes between newly galvanized, partially weathered, and fully weathered surfaces. For newly galvanized steel, acetone wiping may be used after an alkaline wash to remove remaining organic residues; the surface is then etched, sweep-blasted, or treated with a compatible metal conditioner because paint applied directly to zinc without profiling is prone to delamination. For partially weathered galvanized steel, soluble salts from atmospheric corrosion are a primary cause of blistering and must be tested by ISO 8502-6 or equivalent Bresle patch methods; acetone wiping has no effect on chloride or sulfate contamination. Water-break testing per ASTM F22 has limited diagnostic value on weathered zinc because the surface is hydrophilic even when contaminated with soluble salts. The risk of retained acetone in zinc/zinc-iron intermetallic pores is lower than on porous coatings, but forced drying at 20–25 °C for 15 minutes is required before primer application. Adhesion of the final system is usually verified by pull-off per ISO 4624:2023 after the full coating schedule; a typical minimum for galvanized steel is 3–5 MPa, but specification values depend on the coating family and end-use environment.

Acetone cleaning during field maintenance painting requires engineering controls for flammable vapor and operator exposure. The OSHA PEL for acetone is 1000 ppm as an 8-hour time-weighted average, while the NIOSH REL is 250 ppm; the more conservative limit is often adopted because acetone has a low odor threshold and is metabolized rapidly, but eye and respiratory irritation increase with concentration. The flammable range of 2.5–12.8 vol% can be reached quickly in tanks, vessels, and poorly ventilated enclosures; continuous lower explosive limit monitoring and mechanical ventilation at a minimum of 10 air changes per hour are recommended. Equipment must be grounded and bonded per NFPA 30 and 29 CFR 1910.106; plastic containers are not permitted unless they are listed for flammable liquid service. Wipes saturated with acetone are managed as hazardous waste in self-closing metal containers because residual solvent vapors can accumulate and constitute an ignition hazard. Nitrile gloves are not recommended for continuous immersion because acetone can cause swelling; butyl rubber or laminated film gloves provide more consistent protection. The surface temperature during solvent cleaning must remain at least 3 °C above the dew point to prevent condensation, and solvent cleaning should be halted when substrate temperatures fall below 5 °C because evaporation is too slow for reliable coating application. If acetone is used in a paint booth, the air velocity face reading should be maintained between 0.5 and 1.0 m/s for adequate vapor capture in open-face designs.