Paint Foaming: Causes, Microfoam and Additive Fixes

Paint foaming is air held in a liquid paint by surfactant-lined films, and the product examples cited on this page dose its fix at 0.05 to 2.5 % as supplied on total formula, depending on the paint system. Formulators meet it as foam in paint, as bubbles in the paint film, as microfoam in thick or high-solids films and as pinholes where bubbles break late.

Where does the air come from, and which additive releases it? Five air sources run from the grind to the roller and spray gun, and six classes of paint additives remove the air or keep it from forming, from mineral oil and silicone defoamers to air-release grades and moisture scavengers.

Below follow the look of foam, its causes in process order, the additive and process fixes, the air-content test, where a drop from 1.300 to 1.235 kg/L reads as 5.0 % air, and the EU siloxane limits.

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 Does Paint Foaming Look Like From Batch to Dry Film?#

Paint foaming shows as bubbles rising in the batch during grind, let-down and filling, as bubbles in the wet film after roller or spray application, and as gas bubbles from 2K curing reactions. The foam changes shape as it ages: spherical bubbles drain into polyhedral cells as liquid runs out of the films between them (Adams, JCT, 1998).

The table sorts where foam appears by the stage of the paint's life and what the formulator sees at each point.

The objective sign is measured air content, the density lost between unfoamed and foamed paint, set out in the confirmation section.

Why Does Paint Foam? Causes Ranked by Stage#

Paint foams because surfactants stabilize air that enters at four stages, ranked in process order: the surfactant film, air intake from grind to application, zero-VOC surfactant load, and 2K curing and porous substrates. The four causes are listed below.

• Surfactant films: emulsifiers, wetting agents and dispersants that hold each bubble.
• Air intake: stirring, milling, filling and application.
• Formulation: zero-VOC paints that carry more surfactant.
• Cure and substrate: urethane curing gas and air leaving a porous surface.

Surfactants That Stabilize the Foam Lamella#

Surfactants, such as emulsifiers, wetting agents and dispersants, stabilize paint foam: they migrate to the air-liquid interface and stabilize a surfactant-lined lamella, so the bubble survives instead of breaking (Adams, JCT, 1998).

The foam stabilizers are part of the formula: emulsifiers carry the latex, dispersants the pigment and wetting agents the substrate wetting. These additives are covered with surfactants and wetting agents, and each one also lines the bubble walls, so the defoamer choice follows the surfactant load.

Air Sources From the Grind to Application#

Air enters paint at five points named by Adams (JCT, 1998): stirring and milling, filling, 2K curing reactions, porous substrates, and roller and spray application. The five sources are listed below in the order a batch meets them.

• Stirring and milling: air drawn into the batch while the pigment is dispersed.
• Filling: air folded in as the finished paint runs into the container.
• 2K curing reactions: gas formed in the curing film, named by Adams for urethanes.
• Porous substrates: air leaving the pores under the wet film.
• Roller and spray application: air mixed in at the roller or the spray gun.

Two of the five sources sit in the paint plant and three on the job, so the defoamer has to stay active after storage.

Zero-VOC Formulations and a Higher Surfactant Load#

Zero-VOC paints need more surfactant, so a defoamer that worked in an earlier formula may no longer suffice (CoatingsTech, 2015).

The zero-VOC semigloss acrylic of the 2015 CoatingsTech paper shows the scale: 0.50 % defoamer on total formula in the grind, PVC 22.9 %, 100 KU, ICI 140 mPa·s (1.4 P) and VOC below 5 g/L. How coalescents and glycols count is explained in VOC in paint.

2K Curing Reactions and Porous Substrates#

Two-component urethane paints generate gas during cure and porous substrates release air into the wet film, both named as foam sources by Adams (JCT, 1998). Both sources act after the paint has left the can.

In waterborne 2K polyurethane the foam appears at mixing: BYK-011 (BYK product page) is a polymeric defoamer at 1 to 2.5 % as supplied on total formula that prevents foam on adding the curing agent, the highest product level on this page.

Moisture scavengers form a separate additive class for 2K and moisture-cured 1K polyurethane, and they dehydrate the paint rather than break foam. The class is covered under moisture scavengers for 2K polyurethane.

Additive Fixes for Foaming: Defoamer Classes and Example Levels#

Six additive classes fix paint foaming: mineral oil and vegetable oil defoamers, silicone defoamers, polymeric defoamers, air-release additives (deaerators) and, for 2K polyurethane, moisture scavengers, each with a product example level as supplied on total formula.

Defoamers carry hydrophobic actives, such as hydrophobic silica, which makes up at most 15 % of the defoamer mixture (Adams, JCT, 1998). That 15 % is a share of the defoamer, never a level in the paint.

The table compares the six classes by where each fits, a product example level with its basis, and the trade-off to watch.

Each level in the table belongs to one product and its basis, never to the class as a whole. Across the products on this page the examples run from 0.05 % as supplied on total formula, for three waterborne grades, to 2.5 % for the waterborne 2K polyurethane grade, and a formulator ladders from such a starting level with the basis held.

Paint Foaming in Emulsion Paints by PVC and Gloss#

In waterborne emulsion paint the defoamer follows gloss and PVC: mineral oil defoamers suit flat to medium-gloss acrylic, styrene-acrylic and vinyl emulsions, while silicone and polymeric grades cover the high-PVC and gloss ranges. Aliphatic mineral oils cause serious gloss reduction in moderate to high gloss paints, and less hydrophobic oils need higher dosages than more hydrophobic ones (Adams, JCT, 1998). Polyether siloxanes, Si-O-C or Si-C linked copolymers, have little effect on gloss, because the polyether block gives compatibility.

By pigment volume concentration, the product examples in the fixes table cover gloss emulsions of PVC 18 to 25, mineral oil emulsions of PVC 35 to 70, architectural emulsions of PVC 30 to 85 and high-PVC paints of PVC 60 to 85. Two published guide formulations dose the defoamer by weight on total formula: the PCC EU Ecolabel indoor white guide formulation adds 0.2 % in the grind and 0.2 % in the let-down. A distributor guide formulation (Raykem) gives defoamer at 0.1 to 0.3 % by weight of total batch; the full package of both is set out under emulsion paint formulation.

Gloss is the first filter and PVC the second.

Microfoam in Epoxy, 2K Polyurethane and Solventborne Systems: Air-Release Grades#

Microfoam in epoxy, 2K polyurethane and solventborne paints is handled by air-release grades and polymeric defoamers, and the product examples in the table run from 0.05 to 2.5 % as supplied on total formula. The table lists each system with its product example.

The air-release class is chosen by binder and solvent content rather than by gloss. The class is described under deaerators and air-release additives, and three of the five rows of the table are air-release grades.

Clear films add one test: the silicone-free air-release grade can cloud transparent systems, and the polyolefin-solution defoamer of the fixes table asks for a transparency check.

Process Fixes: Defoamer Form and the Split Dose Across Grind and Let-Down#

The process fixes for paint foaming are the defoamer form and the split of the dose: 100 % active compounds withstand the grind, and emulsions lose defoaming power in a bead mill (Adams, JCT, 1998).

The split of the dose follows the batch. The PCC Group formulation guide mixes the defoamer with water and dispersants before the pigments and fillers go in, then adds defoamer again after the HEC solution, water, binder, thickener and biocide. The US 8,293,002 B2 semi-gloss lists its defoamer twice, after the surfactant and before the amine and pigments, and again after the HEUR thickener; the equipment side of both steps is covered in premix, dispersion and let-down.

The four process rules from Adams (JCT, 1998) are listed below.

1. Add 100 % active compound grades to the grind, where they withstand high shear.
2. Add emulsion grades as a post-add under mild stirring, and keep them out of the bead mill.
3. Control the droplet size: higher shear, lower interfacial tension, lower droplet viscosity and higher paint viscosity all give finer droplets, and droplets of 2 to 10 µm defoam well while droplets below 1 µm have hardly any defoaming effect.
4. Choose grades with high interfacial tension for storage stability.

Storage is the step a foam test on the day of manufacture misses: low-interfacial-tension defoamers lose efficiency after storage and renewed shear.

Product data sheets add conditions for their own grades: BYK-017 needs high shear in millbase concentrates, BYK-A 530 goes into the millbase or in under high shear to prevent cratering, and the BYK-1615 silicone emulsion is stored and transported at 0-40 °C (32-104 °F).

How to Confirm Foaming: Air Content by Density#

Foaming is confirmed by air content: % air = (A - B)/A × 100, where A is the density of the unfoamed paint and B the density after shaking or stirring (CoatingsTech, 2015).

As a worked example, a paint that drops from 1.300 kg/L (10.85 lb/gal) to 1.235 kg/L (10.31 lb/gal) after foaming holds (1.300 - 1.235)/1.300 × 100 = 5.0 % air (example values; lb/gal calculated with 1 lb/gal = 119.8264 g/L). No current ASTM or ISO method measures foam in paint, so the density itself is read by ASTM D1475-13(2025) or ISO 2811-1:2023.

The table compares the confirming methods by what they measure, their conditions and their status.

Shaker and Dissolver Methods and Their Link to KU and ICI#

The shaker method foams 150 g of paint at low shear and the dissolver method at high shear, and each reads density right after (CoatingsTech, 2015). The steps of both methods are listed below.

1. Shaker: stir 150 g of paint with 0.5 % defoamer for 5 min at 500 rpm.
2. Shaker: shake 5 min on a Red Devil shaker at the outer clamp, then measure density immediately.
3. Dissolver: run 150 g of paint for 3 min at 3,000 rpm with a toothed disc, then measure density.

Air content from the low-shear shaker method correlates with KU, and air content from the high-shear dissolver method with ICI viscosity. The dissolver result is reliable only below 3,000 rpm for 4 min, because the heat of longer runs lowers the viscosity.

The reading is stable over the first hour: less than 1 % air was lost in the first hour after shaking (formula 758-63). KU is measured by ASTM D562-10(2023) and ICI by ASTM D4287-00(2023), both described under Krebs (KU) and ICI viscosity.

Withdrawn and Misapplied Foam Standards: ASTM D3519, D3601 and D1173#

ASTM D3519 (blender test) and ASTM D3601 (bottle test), both in their 88(2007) editions, were withdrawn in 2013, and ASTM D1173-23, the Ross-Miles method, measures the foaming of surface-active agents rather than of a paint.

D3519 was written for liquids below 3 cSt at 40 °C (104 °F). A specification citing D3519 or D3601 points to a withdrawn method, and a Ross-Miles result describes a surfactant solution, not the paint.

D4, D5 and D6 Residues in Silicone Foam Control: EU Entry 70 Limits#

D4, D5 and D6, the cyclic siloxane residues of silicone defoamers, must each stay below 0.1 % w/w in any mixture placed on the EU market after 2026-06-06 (REACH Annex XVII entry 70 as replaced by Regulation (EU) 2024/1328).

The three are residues of silicone polymers, not additives, and they have been on the Candidate List since 2018-06-27. The limit reaches every silicone grade in the fixes table, and the class itself is covered under silicone defoamers.

The table lists the EU rules that reach foam-control actives in paint.

The para 6(b) derogation covers mixtures "for use as protective coatings (including marine coatings)" only, and an interior wall paint stays under the 0.1 % rule.

How Does Foaming Differ From Cratering and Fish Eyes?#

Foaming is air held in the paint, whereas craters and fish eyes are depressions in the film, and the two meet because an incompatible defoamer, the additive that breaks foam, can cause fish eyes, craters and crawling (Adams, JCT, 1998).

A defoamer must be distinctly incompatible to reach the air interface, but incompatible substances cause fish eyes, craters and crawling in the same film. Too much incompatibility trades foam for craters, and the crater side is covered under cratering and fish eyes.

The table separates foaming from its two neighboring application defects.

When the Foam Fix Causes Craters and Crawling#

Pure silicone oils can cause severe cratering and crawling, and polyether modification improves their compatibility (Adams, JCT, 1998).

Where a silicone grade craters, the next candidate is a polyether-modified siloxane or a silicone-free grade of the fixes table, tested again for air content and film appearance.

Surfactants That Defoam and Surface Additives That Hold Foam#

Two surface-active additive groups sit on either side of foam control: acetylenic diol surfactants act as molecular defoamers, and silicone surface additives need a foam check. The three related additive pages are listed below.

• acetylenic diol surfactants: wetting agents sold as molecular defoamers, not polymeric defoamers.
• tetramethyl decynediol (TMDD): the acetylenic diol behind these grades.
• silicone surface additives: polyether-modified polysiloxanes for flow and slip.

Two data sheets show both sides: Surfynol 104 PG 50 (Evonik TDS), 50 % TMDD in propylene glycol at 0.1 to 1.0 % as supplied on total formula, lists defoaming among its effects, while the BYK-333 product page lists foam stabilization among the properties to test.

Why do I keep getting bubbles in my paint?#

Bubbles keep forming when air from mixing, filling or roller and spray application meets enough surfactant to stabilize it and too little defoamer that persists after storage (Adams, JCT, 1998).

How does a defoamer work?#

A defoamer works by entering the foam lamella and spreading on it, and both steps depend on the surface tension of the paint, of the defoamer and their interfacial tension (Adams, JCT, 1998).

The classes and their selection are set out under how paint defoamers break foam. Silicone antifoams are also used in other industries.