Anti-skinning agents are the additives that stop an air-drying alkyd or oil paint from forming a solid skin in the closed can, and paint formulators choose them from 3 types. The 3 types of antiskinning additives are oximes, non-volatile antioxidants and amino, imine or oxime-free compounds. Two oximes, methyl ethyl ketoxime (MEKO) and acetone oxime, carry a harmonized Carc. 1B H350 classification in the EU, and that classification is why every MEKO-free route on this page is compared with MEKO.
Each type works against the same driers in a different way. Anti-skinning agents are one of the 15 families of paint additives, and this page covers why paint skins, how each type stops it, which type suits which paint and drier system, all 6 anti-skinning substances with their CLP line, the 2 defects the family prevents or causes, the EU and US rules, the 2 published skin tests and how to source a grade.
How we check: regulations and classifications are read at the body that issues them, standards at the standards body, and every additive level names its basis and its source; the full method is in methodology and fact-checking.
What Are Anti-Skinning Agents and Why Do Alkyd and Oil Paints Need Them?#
An anti-skinning agent is a paint additive that keeps an oxidatively drying paint from skinning during production and storage, and the 2014 EU Ecolabel criteria for paints defined "anti-skinning substances" in exactly those terms. Decision 2014/312/EU (definition 12) described them as additives added to the coating materials to prevent skinning during production or storage, and Decision (EU) 2025/2607 has since repealed that text.
Anti-skinning agents belong to solventborne paint. The Danish EPA (2004) describes them as additions to solventborne systems, because waterborne systems are less prone to skinning.
The share of an anti-skinning agent in a finished paint is small: the Danish EPA puts it at a fraction under 1 % of the total formulation. The additive is needed because of the paint driers, the metal catalysts that crosslink the binder with oxygen in the can as well as in the film. Drier metals, their levels on binder solids and the cobalt-free options are set out on paint driers.
Why Does Paint Form a Skin in the Can?#
A skin forms because driers catalyze oxidative crosslinking at the air interface in the can, and skinning is common in oxidatively cured, high-solids and low-VOC paints. The skin is a solid layer on top of the liquid paint in the container, and it forms during production as well as in storage.
Stopping gloss paint from skinning over in a tin depends first on the container. Volatile anti-skinning agents protect the paint only while the container is closed: the Danish EPA (2004) found them ineffective in open or loosely closed containers. The same volatility is the reason they evaporate shortly after application, so the agent that protects the can does not stay in the drying film.
How Do Anti-Skinning Agents Work? Drier Blocking, Radical Trapping and Evaporation#
Anti-skinning agents work in 2 ways: volatile oximes block the drier metal and trap radicals while the can is closed, then evaporate from the film, and non-volatile antioxidants absorb free radicals until they are consumed, after which the film cures. Producer literature describes the oxime step as a temporary complex between the ketoxime and the drier metal that blocks catalysis in the can.
Research gives a more exact picture of the oxime step. Tanase and co-workers (New Journal of Chemistry, 2003, volume 27, pages 854-859) found only weak interactions between cobalt and the oxime, and residual water in the paint can act as a stronger ligand than the oxime. In the same study, radicals added to the oxime C=N bond and formed stable adducts, and these adducts inhibit the radical chains of oxidative drying.
The non-volatile route has its own failure mode. The Danish EPA (2004) found that excess non-volatile agents can significantly slow drying, which follows from the mechanism: the film cures only once the antioxidant is consumed. Antioxidants for this purpose are phenolics, quinones and amines, and each of them stays in the film after application.
3 Types of Anti-Skinning Agents Compared#
The 3 types of anti-skinning agents for paint are oximes, non-volatile antioxidants and amino, imine or oxime-free compounds; oximes evaporate from the film, while antioxidants stay in it until consumed.
The table compares the 3 types by chemistry, mechanism, paint types, one sourced level with its basis, strengths and limits.
| Type | Chemistry and members | How it works | Paint types | Example level (product, basis, source) | Strengths | Limits |
|---|---|---|---|---|---|---|
| Oximes | Ketoximes (MEKO, acetone oxime, methyl propyl ketoxime, methyl isobutyl ketoxime), aldoxime (butyraldoxime), cyclic ketoxime (cyclohexanone oxime) | Temporary complex with the drier metal and radical adducts at the C=N bond in the closed can; evaporates after application | Solventborne air-drying alkyd and oil paints | MEKO 0.32 % of the finished paint (Borchers alkyd control, CTT 2022); MEKO 0.3-0.7 % w/w of total product, historic European level (Danish EPA 2004) | Especially effective with cobalt driers; leaves the film after application | Ineffective in open containers; not efficient with cobalt replacements; MEKO and acetone oxime Carc. 1B |
| Non-volatile antioxidants | Phenolics, quinones, amines; vitamin E in the Danish EPA study | Absorb free radicals; the film cures once they are consumed | Solventborne alkyds with cobalt-free catalysts | See the MEKO-free anti-skinning page | Suited to cobalt-replacement drier systems | Excess slows drying significantly |
| Amino, imine and oxime-free compounds (MEKO-free anti-skinning agents) | Amino compounds in 1,2-propanediol or a fatty acid ester; imine compounds | See the MEKO-free page | Solventborne; one amino product also waterborne | Borchers MEKO-free range, all types (2024 sales tool): 0.15-1.0 % on total weight | Phenol- and MEKO-free products exist | Amino and amido compounds may affect drying more (Danish EPA 2004) |
MEKO-free is a regulatory label rather than a fourth type. One producer groups its MEKO-free options as pentanone oximes, cyclohexanone oxime, amino compounds and imine compounds, so MEKO-free products span the oximes and the amino, imine and oxime-free type.
Which Oximes Are Used, and How Do They Differ?#
Six oximes are used as anti-skins: 4 ketoximes, the aldoxime butyraldoxime and the cyclic ketoxime cyclohexanone oxime, and they differ in volatility and CLP classification. The 4 ketoximes are MEKO, acetone oxime, methyl propyl ketoxime and methyl isobutyl ketoxime. Volatility separates the 2 Carc. 1B members: acetone oxime boils at 135 °C (275 °F) and MEKO at 152 °C (306 °F), so acetone oxime is the more volatile of the two. Butyraldoxime (butyraldehyde oxime, CAS 110-69-0, EC 203-792-8, C4H9NO) is the only aldoxime of the group and is covered on this page.
The CLP classification of the 6 oximes forms a ladder of 3 steps. MEKO and acetone oxime carry harmonized Annex VI entries with Carc. 1B H350. Butyraldoxime carries a harmonized entry without a carcinogen class. Methyl propyl ketoxime, cyclohexanone oxime and methyl isobutyl ketoxime have no Annex VI entry, only the classifications that notifiers report to ECHA, and the full hazard line of each sits in the substance table below.
Lower volatility trades skin protection in the can against drying speed in the film. In the Danish EPA study (2004), cyclohexanone oxime is used only occasionally, and its lower volatility than MEKO can slow drying. The same study named acetone oxime the most promising alternative to MEKO but with a dubious health profile, and acetone oxime has since been classified Carc. 1B in the EU.
Which Non-Volatile and Oxime-Free Types Replace Oximes?#
Antioxidants such as phenolics, quinones and amines, and amino compounds replace oximes where the oxime classification or a cobalt-free drier rules an oxime out. Amino compounds reach the paint dissolved in a carrier, and the carrier decides whether a product suits waterborne or solventborne paint.
The table lists 4 amino-type products, each value tied to its product and the producer's brochure.
| Product (producer, data sheet) | Chemistry and carrier | Paint carrier | Producer statement |
|---|---|---|---|
| Ascinin Anti Skin 0445 (Borchers brochure) | Amino compound in 1,2-propanediol | Waterborne and solventborne | Phenol- and MEKO-free |
| Ascinin Anti Skin 0444 (Borchers brochure) | Amino compound in fatty acid ester | Solventborne | Phenol- and MEKO-free |
| Ascinin Anti Skin 1240 (Borchers brochure) | Amino compound in fatty acid ester | Reduced-VOC oxidatively drying systems and pastes | For reduced-VOC systems |
| Borchi Shield (Borchers brochure) | Amino/oxime compound in fatty acid ester | High-solids alkyds | Slower surface drying for through cure of thick films |
The Danish EPA substitution study (2004) found the success in replacing MEKO rather limited. Among the candidates it tested, amino and amido compounds may affect drying more, and vitamin E had the best health profile.
How to Choose an Anti-Skinning Agent by Paint Type#
Choose an anti-skinning agent by the paint's drying route first: air-drying alkyd and oil paints, wood stains, varnishes and alkyd or epoxy ester primers need one, and the drier package then decides between an oxime and a non-volatile type.
The table matches 5 paint types to the anti-skinning type that fits, a sourced level with its basis and the point to watch.
| Paint type | Type that fits | Example level (product or formulation, basis, source) | Watch-out |
|---|---|---|---|
| Alkyd paint formulation | Oxime with cobalt driers; antioxidant or amino type with manganese or iron catalysts | MEKO 0.32 % of the finished paint (Borchers alkyd control, CTT 2022) | A Carc. 1B oxime at 0.1 % or more classifies the paint |
| Wood stain and varnish formulation | Oxime anti-skin (MEKO use in stains and varnishes listed by Health Canada) | See the wood stain and varnish page | Protection only while the container is closed |
| Primer formulation | Anti-skin in a solventborne alkyd anticorrosive primer | Heubach guide formulation 299-03: Borchi Nox M2 0.25 pbw per 100 (% on total formula), in the let-down | Drier and anti-skin go in as separate additions while stirring in this guide |
| Epoxy paint formulation | Anti-skin in a 1K epoxy ester primer | Heubach guide formulation 734-04: Borchi Nox M2 0.70 pbw per 100 | Cobalt drier in the same formula |
| Emulsion (latex) paint formulation | None as a rule | See the emulsion paint page | Waterborne paints are less prone to skinning (Danish EPA 2004) |
Health Canada lists alkyd paints, varnishes, stains and coatings and antifouling marine paints among the products that contain butanone oxime (MEKO), where it is a formulation component and not the biocide. Paint formulations differ by binder, carrier and drier package. The full package of each type is set out in paint formulations by paint type.
An EU Ecolabel paint narrows the choice further. The Ecolabel excludes ingoing substances classified H350 at 0.010 % w/w or more, which rules out both MEKO and acetone oxime.
Which Anti-Skinning Types Suit Iron and Manganese Catalysts?#
Volatile oximes are especially effective with cobalt but not efficient with cobalt replacements, so non-volatile antioxidant types are the fit for manganese and iron catalysts. Non-volatile antioxidant anti-skins are recommended in cobalt-replacement strategies for the same reason.
The product pairings of one producer show the rule at work: its cobalt-free catalysts are matched with amino-type anti-skins, an amino/oxime product or a cyclohexanone oxime grade in solventborne paint, and with one amino product in waterborne paint. The table lists these pairings.
| Cobalt-free catalyst (producer, source) | Paint carrier | Anti-skin products paired |
|---|---|---|
| Borchi OXY-Coat and OXY-Coat 1310 (Borchers 2024 sales tool) | Solventborne | Ascinin Anti Skin 0444, 0445 or 1240, Borchi Nox 1640 or Borchi Shield |
| Borchi OXY-Coat 1101 (Borchers 2024 sales tool) | Waterborne | Ascinin Anti Skin 0445 |
How Much Anti-Skinning Agent Does a Paint Contain?#
Anti-skinning levels are set per product and formulation: the examples on this page run from 0.15 % on total weight, the low end of one producer's MEKO-free range, to 0.70 pbw per 100 in an epoxy ester primer guide formulation. Dosage arithmetic shows why the old MEKO level no longer works for consumer paint in the EU. The historic MEKO level of 0.3-0.7 % w/w of total product (Danish EPA, 2004) is 3 to 7 times the 0.1 % generic limit at which a mixture is classified Carc. 1B, and the 0.32 % alkyd control is 3.2 times that limit (calculated).
A new anti-skin is dosed by a ladder against a control. In one published ladder, a MEKO control at 0.32 parts ran against a MEKO-free product at 0.25, 0.50 and 1.00 parts per 100 parts base paint. Ladder studies for every family are explained under paint additive dosage.
Complete Table of Anti-Skinning Substances Used in Paint#
The table lists the 6 anti-skinning substances covered on this site, with CAS number, type, function in paint and the CLP hazard line: 4 have their own page, butyraldoxime is covered here and methyl isobutyl ketoxime on the MEKO-free page.
| Substance | CAS | Type | Function in paint | CLP hazard line |
|---|---|---|---|---|
| Methyl ethyl ketoxime (MEKO) in paint | 96-29-7 | Ketoxime | Volatile anti-skin that keeps alkyd and oil paints from skinning in the closed can and evaporates after application | Harmonized CLP Annex VI index 616-014-00-0: H301, H312, H315, H317, H318, H336, H350, H370, H373 (Carc. 1B; Danger), ATP15, Delegated Regulation (EU) 2020/1182 |
| Acetone oxime (acetoxime) in paint | 127-06-0 | Ketoxime | Highly volatile oxime anti-skin for oil and alkyd paints | Harmonized CLP Annex VI index 606-156-00-1: H312, H317, H318, H336, H350, H373 (Carc. 1B; Danger), ATP22, Delegated Regulation (EU) 2024/2564, applicable since 2026-05-01 |
| Methyl propyl ketoxime (2-pentanone oxime) | 623-40-5 | Ketoxime | Volatile oxime anti-skin marketed as a MEKO replacement in air-drying alkyds | No Annex VI entry; ECHA C&L notified classification: H302, H319, H373, H412 |
| Cyclohexanone oxime in paint | 100-64-1 | Cyclic ketoxime | Less volatile MEKO-free oxime for solventborne paint (Borchi Nox 1640) | No Annex VI entry; ECHA C&L notified classification: H302 |
| Butyraldoxime (covered on this page) | 110-69-0 | Aldoxime | Oxime anti-skinning agent | Harmonized CLP Annex VI index 616-013-00-5: H302, H311, H319 (Acute Tox. 3* H311, Acute Tox. 4* H302, Eye Irrit. 2 H319; CLP00, applicable since 2010-12-01) |
| Methyl isobutyl ketoxime (covered on the MEKO-free page) | 105-44-2 | Ketoxime | MEKO-free oxime option | No Annex VI entry; ECHA C&L notified classification: H302, H315, H319 (58.82 %) |
| All other paint additive substances |
MEKO is methyl ethyl ketoxime (butanone oxime, EC 202-496-6, C4H9NO), while MEK is the solvent methyl ethyl ketone (CAS 78-93-3), so the anti-skin is never written as MEK. MEKO is restricted, not banned, in paint: supply of MEKO-containing paint to professional users stays legal.
Which Paint Defects Do Anti-Skinning Agents Prevent or Cause?#
Anti-skinning agents prevent one paint defect, the skin that forms in the closed can, and can cause a second one when they are overdosed or too slow to leave the film: delayed drying.
The table links each defect to its cause and to the anti-skinning fix or risk.
| Defect | Cause | Anti-skinning fix or risk |
|---|---|---|
| Skin in the can | Driers crosslink the paint surface in the container | Oxime in a closed container; antioxidant type with cobalt-free driers |
| Delayed drying (loss of dry in alkyd paint) | Overdosed non-volatile agents; lower-volatility oximes such as cyclohexanone oxime | Ladder the level against a control formulation |
Loss of dry has a second cause outside this family: active metal driers adsorb irreversibly onto pigment surfaces such as carbon black, and the dry time lengthens as the paint ages. Skinning and delayed drying are 2 of the paint defects that an additive choice decides. All other defects and their additive fixes are collected under paint defects.
How Are Anti-Skinning Agents Regulated? CLP, REACH, Ecolabel and US Rules#
Two anti-skinning oximes carry a harmonized Carc. 1B H350 classification in the EU, MEKO by ATP15 (Delegated Regulation (EU) 2020/1182) and acetone oxime by ATP22 (Delegated Regulation (EU) 2024/2564, applicable since 2026-05-01). Neither entry sets a specific concentration limit, so a paint with 0.1 % or more of either oxime is classified as a carcinogen.
The table lists the instruments that reach anti-skinning agents, with their dates.
| Instrument and date | What it requires | Anti-skinning substances affected |
|---|---|---|
| CLP Annex VI: ATP15, Delegated Regulation (EU) 2020/1182 of 2020-05-19; ATP22, Delegated Regulation (EU) 2024/2564, applicable since 2026-05-01; CLP00, applicable since 2010-12-01 | Harmonized classification and labeling of the substance | MEKO (616-014-00-0) and acetone oxime (606-156-00-1), both Carc. 1B; butyraldoxime (616-013-00-5) |
| CLP labeling of paints: mixture limits, Regulation (EC) No 1272/2008 | Carc. 1B H350 at 0.1 % (Annex I Table 3.6.2); H317 at 1.0 % for a Skin Sens. 1 component (Table 3.4.5); EUH208 at 0.1 % when the paint is not H317 (Annex II 2.8) | MEKO and acetone oxime (Carc. 1B and Skin Sens. 1, no specific limit) |
| Restricted and banned substances in paint: REACH Annex XVII entry 28, Appendix 2, Commission Regulation (EU) 2021/2204 of 2021-12-13 | No supply to the general public in mixtures at 0.1 % or more; packaging marked "Restricted to professional users" | MEKO |
| Directive 2004/37/EC (carcinogens and mutagens at work) | Applies to substances meeting the Carc. 1A or 1B criteria | MEKO; acetone oxime from 2026-05-01 |
| EU Ecolabel for paints: Decision (EU) 2025/2607 of 2025-12-17, valid until 2032-12-31 | Sum of anti-skinning agents at most 0.40 % w/w of the final product; Table 6 hazards, H350 included, excluded at 0.010 % w/w unless derogated | MEKO and acetone oxime excluded; methyl propyl ketoxime needs a grade check |
| VOC in paint: Directive 2004/42/EC Art. 2(5); 40 CFR 51.100(s) | EU: organic compound with an initial boiling point of 250 °C (482 °F) or less at 101.3 kPa; US: federal definition with its exempt list | MEKO (152 °C) and acetone oxime (135 °C) are EU VOCs; MEKO is not exempt in the US and counts under 40 CFR 59 subpart D and SCAQMD Rule 1113 |
MEKO is not on the REACH Candidate List (253 entries checked 2026-11-03) and not in Annex XIV, so its EU route is classification and restriction, not authorization. Acetone oxime is registered under REACH and is not on the Candidate List either.
In the US, MEKO is on the TSCA Inventory, active in the 2024 Chemical Data Reporting, and subject to the specific chemical test rule 40 CFR 799.2700. Neither MEKO nor acetone oxime is on the California Proposition 65 list (OEHHA list downloaded 2026-11-03). Canada lists butanone oxime on CEPA Schedule 1 and set a Code of Practice for consumer interior alkyd paints in June 2014; a performance review in July 2020 found the risks not effectively managed. A consultation of 2022-07-15 on new Canadian measures is pending. VOC, biocide, CLP label and restricted-substance rules for every family are on paint additive regulations.
The EU Ecolabel caps the sum of anti-skinning agents at 0.40 % w/w of the final product under Decision (EU) 2025/2607, which repealed Decision 2014/312/EU. Only H317, H412 and H413 are derogated for anti-skinning agents in decorative paints and performance coatings, and water-based aerosol spray paints also derogate H411. Methyl propyl ketoxime shows why the derogation list matters: its notified H373 and H302 are not derogated, so its use at 0.010 % or more needs a supplied grade without those classes, which the supplier SDS shows.
How Is Anti-Skinning Performance Tested? Skin Tests and Drying Time#
Anti-skinning performance is tested in 2 steps: a skin test that stores the paint in closed and open containers at 40 or 50 °C (104 or 122 °F), and a drying-time test for any delay of the film.
The table lists both tests with their protocol and what each shows.
| Test | Protocol | What it shows |
|---|---|---|
| Skin test in closed and open containers (paint storage stability tests) | Danish EPA 2004 (closed and open containers, 40 °C); Borchers CTT 2022 (glass vial, 50 °C, rating 0 to 6) | Time to skin and skin hardness |
| Drying time | ASTM D1640/D1640M-14(2022); ASTM D5895 recorder in the Danish EPA study | Delay of the film caused by the anti-skin |
The 2 published skin protocols differ in container, temperature and rating. The Danish EPA study (2004) put 15 g of paint in a closed polyethylene container and 5 g in an open one, inspected both every second day, and stored samples for 2 weeks at 40 °C (104 °F) for stability. A 2022 producer protocol stores the paint overnight, checks it unopened by tilting after 1 week, eliminates any sample skinned after 24 hours, then holds it in a glass vial at 50 °C (122 °F) for 4 weeks and rates it from 0 (no change) through very thin, thin, medium, hard and very hard skins (1 to 5) to 6 (no skin but gelling). Every other paint test method is described under paint test methods.
Drying time is the second check. ASTM D1640/D1640M-14(2022) covers the drying, curing and film-formation stages of organic coatings, and the Danish EPA study used an ASTM D5895 mechanical drying recorder at 23 °C (73.4 °F) and 50 % relative humidity on an 80 µm (3.1 mils) wet film.
Sourcing Anti-Skinning Agents: Grades, Specifications and Quote Requests#
Anti-skinning agents are bought as liquid oximes, crystalline oximes and amino or antioxidant products in a carrier, and a request names the type or substance, the drier system, the paint type, the annual volume, the packaging and the delivery country. MEKO is a clear, colorless liquid, while acetone oxime is supplied as white crystals that melt at 58-62 °C (136-144 °F).
The table lists 2 MEKO-free grades with their chemistry and the specification their producer publishes; the amino-type products are in the table of non-volatile and oxime-free types.
| Product (producer, data sheet) | Chemistry | Specification or dosing statement |
|---|---|---|
| Borchi Nox 1640 (Borchers brochure) | Cyclohexanone oxime | MEKO-free; producer states no discoloration and no adverse effect on drying time |
| Valirex Noval Next (Umicore product page) | Composition undisclosed | Purity min. 99 %, water max. 0.1 %; dosed one to one in place of MEKO; marine coatings included |
Annual volume follows from the paint output: product mass equals paint output times level divided by 100, so a worked example of 1,000 t of alkyd paint at 0.32 % MEKO on total formula needs 3.2 t of MEKO (calculated). The same arithmetic applies to a MEKO-free product at its own level.
US suppliers deliver an English safety data sheet of 16 sections with the initial shipment and after an update (29 CFR 1910.1200(g)). Sourcing and qualifying producers and distributors is covered on paint additive suppliers and manufacturers.
Two related pages cover regional sourcing and the market.
- Producers and distributors in India: paint additive manufacturers in India
- Market size, segments and leading producers: paint additives market
How Anti-Skinning Agents Interact With Driers#
Anti-skinning agents interact with driers in 2 ways: oximes lose efficiency when cobalt is replaced, and the drier load of the paint sets the reaction the anti-skin must hold back in the can. The 2 related pages are listed below.
- Cobalt-free catalysts: manganese and iron catalysts take volatile oximes out of their best working range, so antioxidant and amino types follow them; see cobalt-free driers.
- Drier dosing: driers are dosed as % metal on binder solids, and metal on binder solids sets the drier load in the can; see paint drier calculator.