Solvent rub test for powder coating cure: what it shows, misses
요약
ASTM D5402-19(2024) counts the solvent double rubs a coating withstands, showing solvent resistance that tracks powder cure but does not prove full cure. As of 15 October 2026 this 2019 edition, reapproved in 2024, is active (ASTM, 2024). It sets no solvent, rub count or pass level, so read results against a cured panel of the same powder.
ASTM D5402-19(2024) is the active edition of the solvent rub practice, last updated on 2 July 2024 (ASTM, 2024).
One powder maker's lab method uses 100 MEK double rubs under a 0.9 kg (2 lb) hammer, rated 0 to 5, where a 3 is typically considered cured (Diamond Vogel, 2020).
One TGIC polyester powder line cures from 20 minutes at 182 °C (360 °F) to 5 minutes at 216 °C (420 °F) peak metal temperature (Diamond Vogel, 2022).
In a 2017 DSC study, an uncured thermal polyester powder coating on MDF scored higher in an MEK double rub test than a cured one (Knoblauch and VanAmburgh, 2017).
This answer covers reading manual and machine solvent rub results on thermoset powder coatings, which crosslink as they cure. It does not cover ethyl silicate zinc-rich primers, which have their own method, or how to run the test. Methyl ethyl ketone (MEK), xylene and isopropanol are flammable solvents; any test setup follows each solvent's SDS and site rules.
What does the ASTM D5402 rub count measure on a powder coating?
The ASTM D5402 rub count measures how many back-and-forth strokes with a solvent-wet cloth a cured organic coating withstands before it is damaged, which reflects its solvent resistance. ASTM D5402-19(2024) describes a technique "for assessing the solvent resistance of an organic coating that chemically changes during the curing process" and allows its use in the laboratory, in the field or in the fabricating shop (ASTM, 2024). The 2019 edition was reapproved in 2024 and is listed as active (ASTM, 2024).
The link to cure is indirect. ASTM explains that coatings which chemically change while curing, such as epoxies, polyesters and urethanes, "become more resistant to solvents as they cure" (ASTM, 2024). Powder coating resins of these types crosslink in the oven, so a well-cured film usually resists more rubs than an undercured one. A writer for the American Coatings Association (ACA) describes the test as a quick way for a production quality lab to simulate the degree of cure by counting the double rubs it takes to damage the surface (ACA, 2025).
The practice leaves the key settings open. ASTM D5402-19(2024) "does not specify the solvent, number of double rubs, or expected test results" (ASTM, 2024), and states that the coating manufacturer may specify the solvent, the number of double rubs and the results needed (ASTM, 2024). A rub count therefore has meaning only together with the solvent, the pressure, the cloth and the film thickness used.
How do you read solvent rub results on powder coated parts?
You read solvent rub results on powder coated parts by rating the damage after the agreed number of double rubs and comparing it with a fully cured panel of the same product. Diamond Vogel, a powder coating maker, advises that results "should always be compared with fully cured panels representing the same product under evaluation" and that the method requires interpretation (Diamond Vogel, 2020). The table below links each result to its meaning and the next check, using that maker's 0 to 5 scale at 100 MEK double rubs; with a milder solvent the same powder rates differently.
Result | Meaning | Action |
|---|---|---|
No mark, no gloss change, no softening (rating 5) | Full solvent resistance at this count; epoxies typically rate 4 to 5 (Diamond Vogel, 2020) | Accept if film thickness and reference panel match; confirm by DSC when the powder or oven changes |
Slight marring or softening by fingernail (rating 4) | Typical of high-gloss TGIC polyesters, which tend to soften but keep their gloss (Diamond Vogel, 2020) | Accept if the cured reference panel shows the same softening |
Gloss loss, film intact (rating 3) | Typically considered cured in this method; semi-gloss products typically rate 3 (Diamond Vogel, 2020) | Pass; if the reference panel keeps its gloss, check the oven profile |
Color on the cloth with gloss loss (rating 2) | Generally undercure, except textures and metallics, which typically show some transfer (Diamond Vogel, 2020) | For smooth solid colors, check peak metal temperature and time, then retest |
Film appears melted, partly dissolved or rubbed to bare metal (rating 0 to 1) | Severe undercure; these ratings typically appear early in the test (Diamond Vogel, 2020) | Hold the lot and review the cure schedule with the powder supplier |
Rub-through to substrate before the agreed count | Reported as a failure (ACA, 2025) | Measure dry film thickness first; a thin film exposes the substrate sooner (ACA, 2025) |
The key row is color on the cloth: on a smooth solid color it usually means undercure, while on a texture or metallic it is normal (Diamond Vogel, 2020). Film thickness changes every row. Thicker coatings give higher rub numbers because the substrate takes longer to show (ACA, 2025), so compare parts and reference panels at the film thickness range on the technical data sheet.
Worked example (illustrative values): a smooth gloss polyester part shows color on the cloth and gloss loss, a rating of 2, after the agreed count, while the cured reference panel of the same powder rates 4. Its film thickness is within the data sheet range. Record the solvent, weight, cloth and rub count with each rating, since the same maker stresses keeping them constant (Diamond Vogel, 2020). The pattern points to undercure, so the next check is the oven profile against the powder's cure schedule, then a DSC glass transition run if the profile looks right.
How many double rubs show a powder coating is cured?
No fixed number of double rubs shows that a powder coating is cured; the count is agreed between powder supplier and user for each product, solvent and film thickness. ASTM D5402-19(2024) sets no number (ASTM, 2024), and the ACA article states that "the producer and user agree on the solvent and the number of double rubs based on formulation chemistry" (ACA, 2025). The same article notes that the film thickness range, required number of rubs and type of solvent are listed on the technical data sheet (ACA, 2025).
The machine method agrees. ASTM D7835/D7835M-26, the mechanical rubbing machine method, states that the solvent and the number of double rubs "should be agreed upon between manufacturer and user for any given coating system, thickness, and application" (ASTM, 2026). It adds that the test solvent has a significant effect on the number of double rubs measured (ASTM, 2026).
Published counts are examples, not limits. Diamond Vogel's lab method uses 100 MEK double rubs unless noted, with a 0.9 kg (2 lb) hammer and a set towel (Diamond Vogel, 2020); a study of UV-cured powder coatings used 50 MEK double rubs (Knoblauch and VanAmburgh, 2017). PPG suggests starting at 20 double rubs of a 75/25 volume blend of 99% isopropanol and xylene, failing on color removal from the cloth, and calls MEK aggressive toward most powder coating films (PPG, n.d.). Diamond Vogel points readers to Powder Coating Institute (PCI) Recommended Procedure #8 (Diamond Vogel, 2020). For coil coated metal, the European method EN 13523-11:2019, current in the UK as BS EN 13523-11:2019, rubs at a specified load and counts one forward and one backward rub as a double rub (BSI, 2019). A count copied from one of these into another powder's specification has no basis unless the supplier confirms it.
Why can a powder coating pass a solvent rub test but still be undercured?
A powder coating can pass a solvent rub test but still be undercured because solvent resistance can develop before the crosslinking that service properties need is complete. ASTM D5402-19(2024) states this directly: "the level of solvent resistance by itself does not indicate full cure and some coatings become solvent resistant before they become sufficiently cured for service" (ASTM, 2024).
Keyland Polymer compared MEK double rub results with differential scanning calorimetry (DSC) on thermal and UV-cured powder coatings on MDF panels (Knoblauch and VanAmburgh, 2017). One white polyester powder coating showed a sharp exotherm starting at about 140 °C in the first DSC heat, which marks unreacted material, yet it scored higher in the MEK test than a cured sample (Knoblauch and VanAmburgh, 2017). A UV-cured yellow epoxy also scored high in the MEK test while its DSC showed unreacted photoinitiator and a final glass transition temperature of 68 °C (Knoblauch and VanAmburgh, 2017). The authors call both results false positives if the MEK test is used to evaluate cure (Knoblauch and VanAmburgh, 2017).
Three conditions make a false pass more likely. First, the authors describe proxy tests as methods that "only evaluate by observation the surface condition of the coating" (Knoblauch and VanAmburgh, 2017). Second, a thick film raises the count (ACA, 2025). Third, film chemistry, oven air velocity and film thickness also change solvent resistance (ASTM, 2026). This suggests a pass is strongest evidence when film thickness and the oven profile are also in range.
How well does the solvent rub test predict powder coating service?
The solvent rub test predicts powder coating service only indirectly, because it separates cured from clearly undercured films but does not measure the properties that fail in service. Diamond Vogel describes the method as "designed to differentiate cured and uncured powder coatings" and notes that it requires interpretation (Diamond Vogel, 2020). The ACA article reports that the manual method shows a 95% confidence limit for reproducibility between round robin operators, with very thin 6H pencil hardness films at 90% (ACA, 2025). That is evidence of reproducibility between operators, not of service life.
We found no public study that links a powder coating's rub count to its field life. The service properties that a powder data sheet lists come from other tests. One TGIC polyester line lists adhesion by ASTM D3359, impact by ASTM D2794, flexibility by ASTM D522, humidity by ASTM D2247 and salt spray by ASTM B117 for its cured film, and states that film properties depend on gloss, pigmentation, film thickness and cure temperature (Diamond Vogel, 2022).
The test is strongest as a release check on a process that has already been qualified. During development, ACA describes glass transition methods such as DSC as the usual way to set curing temperature and time, and the rub test as the production method correlated with that laboratory work (ACA, 2025). Used that way, a drop in rub rating can flag an oven or thickness change early.
Common mistakes when reading solvent rub results on powder coatings
The most common mistakes when reading solvent rub results on powder coatings come from comparing counts made under different conditions or treating a pass as proof of full cure. Five recur in the sources.
First, using the wrong standard. ASTM D4752-20(2024) measures the MEK resistance of ethyl silicate (inorganic) zinc-rich primers, which cure by reaction with moisture, and its own scope names Practice D5402 as "the preferred method for organic coatings" (ASTM, 2024). Its rating scale does not apply to powder coatings.
Second, comparing counts across methods. Diamond Vogel warns that "the pressure applied, number of rubs, and the type of solvent used will affect the results" (Diamond Vogel, 2020). A finger-pressure count and a hammer or machine count of the same panel are not interchangeable. For repeatable counts, ASTM D7835/D7835M-26 uses a machine that gives consistent stroke length, rate, pressure and contact area (ASTM, 2026).
Third, skipping the reference panel. Coatings do not all have the same solvent resistance, and the procedure sometimes has to be modified for a specific product (Diamond Vogel, 2020).
Fourth, reading color transfer on textures and metallics as undercure. Textures typically show a little color transfer because of the rough surface while the film stays intact, and metallics typically show a noticeable change in appearance with metallic transfer (Diamond Vogel, 2020).
Fifth, ignoring film thickness. Thicker coatings give higher counts (ACA, 2025), so a thick, undercured part can outscore a thin, cured one.
Which tests confirm powder cure alongside the solvent rub?
The tests that confirm powder cure alongside the solvent rub are a DSC glass transition check, an oven profile against the powder's cure schedule, and the mechanical tests on the data sheet. ASTM E1356-25, the active edition, covers the assignment of glass transition temperatures by DSC or differential thermal analysis (ASTM, 2025), and ASTM states that the glass transition may give information about thermal history, processing conditions and progress of chemical reactions (ASTM, 2025).
In practice the DSC check uses a heat-cool-heat cycle. The first heat cures any remaining unreacted material, and the second heat measures the glass transition temperature and shows any further curing reaction (Knoblauch and VanAmburgh, 2017). An exotherm in the first heat that is absent in the second marks an undercured film (Knoblauch and VanAmburgh, 2017). The authors of that study call 70 to 75 °C a reasonable minimum glass transition range for a cure determination on the coatings they tested (Knoblauch and VanAmburgh, 2017).
The oven profile checks the cause. Cure schedules are given as time at peak metal temperature, such as 10 minutes at 199 °C (390 °F) recommended for one TGIC polyester line (Diamond Vogel, 2022), so an oven profile with a data logger shows whether the part reached the schedule. When rub results, DSC and oven data disagree on a lot, Lubinpla can work the case from the powder data sheet, test records and oven history and return one cited document for your review.
Key takeaways
ASTM D5402-19(2024) measures solvent resistance, which tracks powder cure but does not prove full cure (ASTM, 2024).
No universal rub count exists: the solvent, the number of double rubs and the pass level are agreed with the powder supplier for each product and film thickness.
Read results against a fully cured reference panel of the same powder; color transfer on textures and metallics is normal.
Thick films raise the count, and a published DSC study found an uncured powder coating that outscored a cured one in the MEK test.
Confirm cure on new powders or ovens with DSC glass transition (ASTM E1356-25), an oven profile against the cure schedule and the data sheet's mechanical tests.
ASTM D4752 is for ethyl silicate zinc-rich primers, not powder coatings.