Why Do Craters, Fish Eyes or Bubbles Appear in Paint?

Short answer

Craters, fish eyes or bubbles in paint are most often caused by surface contamination or by an underlying layer that has not flashed off sufficiently. Silicone, grease, wax, polish residues, unsuitable cleaner, dust or other contamination may remain on the surface and repel the newly applied paint or clear coat.

Another common cause is applying clear coat over paint that has not flashed off enough. Residues of thinner and solvents then try to escape through the clear coat layer and can create small craters, pinholes, bubbles or damaged areas in the surface.

What are fish eyes?

Fish eyes are small circular defects in paint where the colour or clear coat pulls away from the centre toward the sides. A small crater, dent or round mark then appears on the surface where the coating does not hold evenly.

It typically looks as if the paint “runs away” from a certain spot or refuses to stick. The most common cause is dirt or grease on the surface that repels the freshly applied material.

What is the difference between a crater, a fish eye and a bubble?

In practice, these defects can look similar, but the cause may be different.

Fish eye: the coating pulls away around a contaminated spot, often due to silicone, grease, wax or polish.
Crater: a small dent or damaged spot in the paint, which can be caused by contamination or solvents escaping from the lower layer.
Bubble: a small raised area or hole caused by air, solvents or volatile substances escaping from the substrate or lower layer.

That is why it is important not to judge only the appearance of the defect, but also when it appeared, on which base and at which stage of painting.

Most common causes of craters, fish eyes and bubbles

silicone on the surface,
grease or oil,
wax, polish or protective products,
residues of unsuitable cleaning products,
poor or insufficient degreasing,
using gasoline or thinner for degreasing,
sanding dust,
contamination from the hose, compressor or spray gun,
moisture or oil in compressed air,
touching the surface with bare hands after degreasing,
colour layer that has not flashed off sufficiently,
paint or clear coat layers applied too heavily,
too short waiting time between individual layers.

Surface contamination as the main cause

The most common cause of fish eyes is surface contamination. Even a very small amount of silicone, grease, wax or polish can cause the coating not to flow correctly in that area.

Paint or clear coat will not stay on the contaminated spot, moves away to the sides and creates a small circle or crater. This problem can appear during application of the base colour, effect paint or only when clear coat is applied.

Why is silicone such a problem?

Silicone is very risky in painting. It can be present in polishes, protective products, plastic care sprays, interior cleaners, old waxes or some universal cleaning products.

Even a small amount of silicone can cause a clear fish eye. The coating does not want to bond to the surface in that spot and pulls away to the sides. That is why it is important to use a suitable silicone remover before painting and to work with clean wipes.

Unsuitable use of degreasers

A very common mistake is using an unsuitable product for degreasing. For surface preparation before painting, we do not recommend using gasoline, technical gasoline, standard thinner or universal cleaners.

Gasoline can leave an oily film or residues on the surface, which can reduce adhesion and cause paint defects. Thinner is also not a suitable degreaser. It can attack the surface, soften older layers, leave residues or simply spread contamination across the surface instead of removing it.

For degreasing before painting, we recommend using only suitable silicone removers designed for automotive paint surface preparation. These are made to help remove grease, silicone and dirt without unnecessarily damaging the prepared base.

How to degrease the surface correctly

Correct degreasing is not just wiping the surface with any product. The important thing is to actually remove contamination from the surface, not just spread it around.

use a suitable silicone remover,
work with clean paint preparation wipes,
do not use gasoline or standard thinner,
wipe the surface gradually in smaller sections,
remove contamination with a clean dry wipe,
do not touch the surface with bare hands after degreasing,
before painting, allow the surface to flash off and dry completely.

If you use one dirty wipe for the whole surface, you may only spread grease or silicone over a larger area. That is why it is important to use clean wipes and replace them when needed.

Craters and bubbles caused by paint that has not flashed off enough

Craters or small bubbles in clear coat do not have to be caused only by surface contamination. A common cause is also applying clear coat over paint that has not flashed off sufficiently.

Residues of thinner and solvents can remain in the colour layer. After clear coat is applied, these volatile substances try to escape through the new clear coat layer. If they do not have enough space to evaporate gradually, they can create small craters, pinholes, bubbles or damaged spots in the clear coat.

That is why it is important to let the colour layer flash off correctly before applying clear coat. The surface should be even, dry to the touch and ready for the next layer according to the recommended process. If clear coat is applied too early, the problem may only appear after the clear coat has been applied.

Why is a colour layer that is too heavy a problem?

A colour or effect paint layer that is too heavy can contain a larger amount of solvents. These need more time to flash off. If clear coat is applied too soon over such a layer, solvents can become trapped inside the paint system.

Trapped solvents then try to escape and can disturb the fresh clear coat. The result can be bubbles, small craters, pinholes, cloudy marks or an overall unstable surface.

How to tell whether it is contamination or poor flash-off

It is not always easy to distinguish the cause, but some signs can help.

Fish eyes immediately during spraying: often point to silicone, grease or other surface contamination.
Bubbles or small craters after clear coat: may point to paint that has not flashed off enough or a lower layer that was applied too heavily.
Defects only in some areas: often relate to local contamination, touching with hands or polish residues.
Defects across the whole surface: may relate to generally poor degreasing, unsuitable cleaner, bad compressor air or unsuitable application conditions.

Contamination from the compressor or spray gun

When painting with a spray gun, the air system can also be the source of the problem. If there is water, oil or other contamination in the compressed air, it can get into the paint or clear coat and cause craters, fish eyes or other defects.

Clean compressed air, a water and oil separator, clean hoses, a clean spray gun and proper equipment maintenance are therefore important. Even a perfectly prepared surface can be ruined by contamination from the air or spray gun.

What to do if fish eyes appear during painting

If fish eyes appear already during application, simply adding more clear coat usually does not help. The coating will continue to pull away from the contaminated areas and the defect may remain visible or become worse.

In this situation, it is better to stop the application, let the material flash off and choose the repair based on the extent of the defect. In most cases, after curing, the surface must be sanded, thoroughly cleaned, degreased with a suitable silicone remover and recoated.

What to do if bubbles or craters appear after clear coat

If bubbles or small craters appear only after applying clear coat, the problem may be in the lower layer. It is often paint that has not flashed off sufficiently, a layer that was applied too heavily or trapped solvents.

In this case, it is not suitable to immediately cover the problem with another layer of clear coat. Another layer can trap the remaining solvents even more and reduce the stability of the whole paint system. It is better to let the clear coat cure, assess the surface and repair it by sanding and reapplication if needed.

How to prevent the problem

clean and degrease the surface thoroughly before painting,
use only a suitable silicone remover,
do not use gasoline, technical gasoline or standard thinner as a degreaser,
do not use dirty or repeatedly contaminated wipes,
do not touch the surface with bare hands after degreasing,
allow each layer to flash off correctly,
do not apply paint or clear coat layers too heavily,
apply clear coat only over paint that has flashed off correctly,
when using a spray gun, check the cleanliness of the air, hoses and gun,
work in a clean environment without silicone, oil, wax and dust.

What to avoid before painting

cleaning the surface with gasoline,
degreasing with standard thinner,
using polishes, waxes or silicone products before painting,
touching the surface with bare hands after degreasing,
applying clear coat over paint that has not flashed off enough,
applying heavy layers without sufficient waiting time,
painting in a dusty, greasy or contaminated environment,
using a contaminated spray gun, hose or compressor air.

When does the surface need to be sanded?

If craters, fish eyes or bubbles are clearly visible, it is usually necessary to let the coating cure and sand the surface. Simply spraying over the defect often does not remove it, because the cause remains in the surface or lower layer.

After sanding, the surface must be thoroughly cleaned again, degreased with a suitable silicone remover, allowed to flash off and only then recoated.

Summary

Craters, fish eyes or bubbles are most often caused by surface contamination or an underlying layer that has not flashed off sufficiently.
Silicone, grease, wax, polish or an unsuitable cleaner can cause the coating to pull away from the surface.
Gasoline and standard thinner are not suitable for degreasing before painting.
For degreasing, we recommend a suitable silicone remover.
Craters and bubbles can also appear when clear coat is applied over paint that has not flashed off enough.
Residues of thinner and solvents try to escape through the clear coat and can create small holes, bubbles or craters.
Before applying clear coat, the colour layer must be allowed to flash off correctly.
If the defect appears, simply adding another layer usually will not solve it.
Visible defects usually need to be sanded after curing, degreased again and recoated.