Enamel Coating Defects: Fish Scale, Blistering and Pinholes

Enameled steel is a system in which glass and metal work in harmony. When the system breaks down, the defect shows on the surface, but its cause is often somewhere you can't see: in the sheet's carbon and hydrogen content, in a residue from pretreatment, or in the last thirty minutes of the furnace exit curve. Below I go through the enamel defects most often encountered in the field, from symptom to root cause. Enamel is not a paint; it is a glassy layer fused onto metal at 780–850 °C, so its defects speak a different language from paint defects.

Fish Scaling

If you see small, crescent-shaped flakes lifted at the edges on the surface, it is almost always fish scaling. The defect may appear not right after firing but hours or even days later; this delay is the first clue that sets it apart from other defects.

The mechanism has to do with hydrogen. Steel dissolves hydrogen during acid cleaning (pickling) and firing. When the sheet is hot, hydrogen solubility is high; as it cools, it drops. Hydrogen released while the glass layer has already solidified and become gas-tight accumulates at the glass–metal interface, builds pressure and lifts the enamel locally. That is the flaking.

Prevention comes from three directions. First, material: low-carbon, decarburized enameling steel releases hydrogen more easily and does not let it accumulate at the interface. Second, frit: a fish-scale-resistant ground coat acts like a micro-porous reservoir for hydrogen, letting the gas through without accumulating it. Third, regime: not keeping pickling time longer than necessary and not cooling too fast after firing.

In the field, rather than waiting to see fish scaling, it is also possible to accelerate it. A hydrogen degassing step that holds the fired part at a low temperature (typically in the 100–150 °C range) for a few hours makes any flaking appear while the part is still on the line; that is better than it popping off a product that has already reached the customer. This does not remove the root cause, but it gives an early warning within production.

Blistering

A blister forms when gas from within swells the surface while the glass layer is still fluid. The difference from fish scaling is that a blister occurs at the moment of firing, while the layer is still soft; it usually leaves a hollow dome or a burst crater.

The gas can have more than one source: moisture in an insufficiently dried wet enamel slip, reaction products left from pretreatment, incompatibility between ground coat and cover coat, or carbon oxidation from the steel itself. High-carbon sheet gives off its carbon as CO/CO₂ at around 800 °C; this gas causes blisters as it passes through the still-liquid glass layer.

Pinholes

A pinhole is a small, punctured version of a blister. Instead of swelling the layer, the gas punches through it, leaving an opening the size of a pin tip. In enamels that act as a barrier, such as water heater inner tanks, pinholes are a critical defect, because a single hole is enough to open the door for corrosion to reach the metal.

• Thickness: Gas punches through an overly thin ground coat more easily. For the ground coat, a thickness giving adequate coverage is typically targeted; the exact value is stated in the technical datasheet of the powder enamel or wet enamel system.
• Firing: Underfiring does not allow the glass to flow fully and seal itself; the hole stays open. Overfiring, on the other hand, can thin the glass too much and open new holes.
• Surface: Local contamination or an oxide island on the sheet concentrates gas release at that point.

Black Specks / Dimples

Dark specks appearing on white or light-colored enamel usually point to two causes. The first is contamination: oil, rust or foreign particles getting onto the surface before the ground coat is applied carbonize during firing and leave a dark mark. The second is the steel's own carbon; in high-carbon areas a local reaction surfaces as a black speck or dimple.

The reddish-brown specks known as copperheads are a separate subcategory and are usually associated with iron/copper contamination. The common lesson of all of them is the same: the cleaner and more homogeneous the substrate under the enamel, the more spotless the surface. Rinse quality on the pretreatment line is directly decisive here; when the conductivity of the final rinse water rises, salt residue remains on the surface and comes back as stains during firing.

Telling a black speck from a blister is also important in practice. If the speck is flat and inside the layer, it is contamination; if it is the burst center of a blister, it is an outgassing problem. Holding the part at an angle to the light and checking whether the surface is raised is often enough to tell the two apart.

Outgassing in Castings

When cast iron is enameled, the nature of the problem changes. A casting has pores on and just beneath its surface; air and gas trapped in these pores expand during firing and try to escape through the glass layer. The result again belongs to the blister and pinhole family, but the source is not hydrogen in the sheet but the porous structure of the casting.

The practical countermeasure for castings is to anneal the part before enameling (a burn-off/degassing anneal) to drive out the gas in the pores beforehand, and often to work with a wet enamel system. On castings with complex geometry, the dipping/flow coating flexibility of wet enamel gives safer coverage than powder application.

Root Cause: Steel + Firing

Most of the defects above come down to two variables: the metal underneath and the firing–cooling regime.

On the metal side, carbon and hydrogen are decisive. High carbon generates gas during firing (blisters, black specks); trapped hydrogen lifts flakes during cooling (fish scaling). Decarburized enameling steel reduces both from the start: the carbon has already been removed, and hydrogen passage is made easier by the sheet's grain structure. That is why much of enamel quality lies more in sourcing the right sheet than in gun settings.

On the firing side, both extremes are dangerous. Underfiring does not let the glass flow and seal itself, leaving holes and dull spots. Overfiring degrades the glass, causing color shift and new gas blisters. Time must be counted from the part's own temperature, not the furnace air; a thick-section casting and a thin sheet in the same furnace for the same time will not give the same result. Cooling is also part of the regime: cooling too fast increases both thermal stress and the risk of fish scaling. For furnace and regime details, the temperature and cooling sections of the enamel firing furnace article are a good place to start.

Diagnostic Table

The table below offers a quick path from the symptom observed in the field to the likely root cause and a practical countermeasure. The exact recipe varies by part, frit and steel; the frit manufacturer's technical datasheet and field measurement are the reference.

Symptom	Likely cause	Countermeasure
Crescent-shaped flakes, delayed lifting after cooling (fish scaling)	Hydrogen trapped in the steel accumulating at the interface during cooling; sheet with high carbon/hydrogen content	Decarburized enameling steel (EN 10209, DC04EK); fish-scale-resistant ground coat; limit pickling time; don't cool too fast
Hollow dome / burst blister, at the moment of firing	Moisture in wet slip, pretreatment residue, carbon oxidation; gas escaping while the layer is fluid	Dry wet enamel fully; correct pretreatment and rinsing; low-carbon sheet; ground coat–cover coat compatibility
Holes the size of a pin tip (pinholes)	Gas punching through a thin/underfired layer; local surface contamination	Adjust ground coat thickness per the TDS; optimize the firing regime; improve surface cleanliness
Dark specks / dimples (black specks)	Oil/rust/particle contamination or locally high carbon	Improve pretreatment and rinsing; check sheet quality; inspect the surface before coating
Widespread blisters/pinholes on castings	Expansion of gas trapped in pores on the casting surface	Degassing anneal before enameling; wet enamel system; controlled heating curve
Color shift, dull/overly glossy spots	Underfiring or overfiring; part temperature deviating from target	Count time from part (metal) temperature; adjust the furnace profile to part mass

When trying to fix a defect, it is important to isolate one variable. If you change the steel, the frit and the furnace curve all at once, even if the result improves you won't learn which one worked. Test one at a time on sample parts and document the result. Enamel defects are repeatable; once the root cause is correctly caught, you get a clean surface with the same recipe for a long time.