You pull a freshly coated panel and the surface is pocked with small craters, each a bare ring where the paint pulled back from a spot it would not wet. The batch ran fine yesterday. The one thing that changed is the new defoamer.
The short version: fisheyes, also called cissing, are craters where the wet film dewets from a low-surface-energy spot. The contaminant is almost always a silicone or an oil. The trap for formulators is that the silicone defoamer added to kill foam is a frequent cause, when the grade is wrong or the dose is too high. The fix is a recoatable, polyether-modified silicone or a silicone-free defoamer, dosed low and added on the grind.
What is a fisheye, and why does it form?
A coating wets a surface only when its surface tension sits at or below the surface energy of what it lands on. A fisheye forms when one tiny spot has a far lower surface energy than the film around it. Polydimethylsiloxane (PDMS) sits near 20 mN/m, while a waterborne paint runs higher, so the film flows away from any PDMS-rich spot and leaves a crater. The same physics applies to oil, wax, and the silicone in hand lotion or a mold release on the substrate.
When the defoamer is the cause
Silicone is the strongest foam knockdown precisely because it is insoluble and low in surface tension. That incompatibility is the working principle: the droplet enters the foam film and ruptures it. Push the dose too high, or pick a non-recoatable grade, and the same incompatibility turns against the finish. Excess silicone migrates to the coating surface, craters the film, and kills intercoat adhesion so the next coat will not hold. A defoamer that controls foam beautifully in the can can still ruin the finish on the wall.
How to stop fisheyes without losing foam control
You do not have to choose between foam control and a clean finish. Match the grade to the system.
| Defoamer grade | Use it when | Recoat behavior |
|---|---|---|
| Recoatable polyether-modified silicone | most waterborne systems needing strong knockdown | recoatable; tuned to break foam without surfacing |
| Silicone-free (polymeric or mineral-oil) | multi-coat and high-gloss work where any silicone risk is unacceptable | safest for recoat and adhesion |
| Standard (non-modified) silicone | single-coat, foam-critical jobs with no recoat | strongest knockdown, highest fisheye risk |
Then three habits keep the defect away: add the defoamer on the grind so high shear distributes and compatibilizes it; dose at the lowest level that holds foam; and rule out substrate contamination before you blame the can.
Fisheye, macrofoam, and microfoam: tell them apart
Three different defects get blamed on each other. A fisheye is a dewetting crater from a contaminant. Macrofoam is the visible bubbling on pour and roll, which most defoamers break easily. Microfoam is fine entrained air that does not rise fast enough to escape the film and dries in as pinholes and craters. They need different answers, and the bubble side is covered in why your paint bubbles.
When it is not the defoamer
Sometimes the can is innocent. Silicone spray, mold release, wax, oil, or hand lotion on the substrate or in the spray line will craters an otherwise good paint. If a fresh batch fisheyes on one substrate but not another, clean the surface and the equipment before you reformulate. Even hydrophobic silica carried over from an upstream step can do it.
Choosing and buying a defoamer for waterborne coatings
The right grade depends on your binder, your gloss target, and how many coats you run. A single-coat, foam-critical system may run a standard silicone well. A multi-coat, high-gloss system is safer on a recoatable silicone-polyether or a silicone-free grade. RawSource carries silicone antifoam emulsions plus silicone-polyether and silicone-free defoamers, so the answer can match your line instead of whatever one supplier happens to make. The trade-offs are laid out in silicone vs. organic defoamers, and the fit across coatings follows the same logic.
For a new formula, trial a sample at two or three dose levels on your actual substrate before you commit to a drum.
Frequently asked questions
What causes fisheyes in paint?
A spot of low surface energy, usually silicone or oil, that the wet film cannot wet. The paint flows away from it and leaves a crater. The contaminant can sit on the substrate or in the paint, and an over-dosed or wrong-grade silicone defoamer is a frequent source.
How do I fix fisheyes once they appear?
On a wet film you generally cannot rescue the coat; sand back and recoat after removing the contamination. In the formula, switch to a recoatable or silicone-free defoamer, lower the dose, and add it on the grind. On the line, clean silicone and oil off the substrate and out of the spray equipment.
Is silicone defoamer bad for paint?
No. The right silicone grade at the right dose is one of the best foam-control tools available. The problem is over-dosing or using a non-recoatable grade in a system that has to recoat. Recoatable polyether-modified silicones exist for exactly this reason.
What does a “recoatable” defoamer mean?
A defoamer built with controlled incompatibility: strong enough to break foam, but tuned so it does not migrate to the surface and block the next coat. It is the usual choice for multi-coat waterborne systems.
How much defoamer should I add?
Start at the low end of the supplier’s range and work up only until foam is controlled. Over-dosing wastes product and raises the fisheye and recoat risk. Validate the dose on your own formula and substrate; published ranges are a starting point, not a spec.
Where a silicone-based defoamer suits the coating system, RawSource supplies Silicone Antifoam Emulsion in bulk to validate against your own formula and substrate.
Editorial note. This article is general guidance for coatings formulators and applicators, written for industrial and professional use. Dose and surface-tension figures are typical literature values and starting points to validate by trial on your own system, not guaranteed specifications or performance promises. Confirm grade suitability and consult the product Safety Data Sheet (SDS) before use. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
