Why Is Paint Peeling or Not Sticking to the Surface?

Short answer

Paint usually peels or does not stick because the surface was not prepared correctly. The surface may have been greasy, dusty, wet, poorly degreased, incorrectly sanded, or the wrong base or primer may have been used. For good adhesion, the surface must be clean, dry and correctly roughened.

The problem can occur not only when the surface has not been sanded enough, but also when it has been sanded too smooth. Paint needs a suitable mechanical surface to bond to. A surface that is too smooth or polished can lead to poor adhesion and later peeling.

Why does paint start to peel?

Peeling means that one layer in the paint system does not have enough adhesion to the layer underneath. The problem may be between the original surface and the first layer, between primer and filler primer, between filler primer and paint, between old paint and new paint, or between newly applied layers.

If one layer does not bond properly to the surface below, the paint may later start peeling, cracking, lifting, blistering or separating from the surface under mechanical stress.

What does it mean when clear coat peels together with the colour layer?

If only the final clear coat layer is peeling, the problem may be adhesion between the colour layer and clear coat, for example due to too long a waiting time, surface contamination or an unsuitable process before applying the clear coat.

However, it often happens that the clear coat peels together with the colour layer or even with other layers underneath. In this case, the problem is usually not in the final clear coat layer, but much deeper in the paint system.

This means that one of the lower layers did not have enough adhesion to the base. The problem often starts right at the beginning, for example when painting over a poorly prepared surface, untreated plastic without plastic primer, an unsuitable primer, glossy old paint, or a surface that was not properly sanded and degreased.

That is why, when peeling occurs, it is important to check exactly where the paint is separating. If the whole paint system peels away down to the first layer, simply applying another clear coat will not solve the problem. Poorly adhering layers must be removed down to a stable surface, and the whole process must be repeated correctly from the first layer.

Most common causes of poor adhesion

• insufficiently sanded surface,
• surface sanded too smooth or polished,
• grease, silicone, wax or residues of cleaning products,
• sanding dust,
• moisture on the surface,
• painting over an unsuitable or incompatible base,
• missing correct primer for the material,
• painting untreated plastic without plastic primer,
• insufficient flash-off time between layers,
• layers applied too heavily,
• low temperature during application,
• putting stress on fresh paint too early.

Why is sanding so important?

Sanding is not only about creating a smooth surface. Its main purpose is to prepare the surface so the next layer can bond to it correctly. Paint, filler primer or primer need a properly roughened surface that creates mechanical adhesion.

If the surface is glossy, smooth or not roughened enough, the new layer may not bond properly. The paint may look fine at first, but later it can start peeling or lifting.

Why is sanding too smooth also a problem?

A surface that has been sanded too finely or polished can be too smooth for the next layer. The paint then does not have enough mechanical grip, and adhesion may be weaker.

A common mistake is trying to make the surface as smooth as possible at all costs. In painting, however, smoother does not automatically mean better adhesion. The surface must look smooth, but at the same time it must be properly roughened for the next layer.

How to recognise a poorly prepared surface

A risky surface is mainly one that is glossy, greasy, dusty, wet or slippery to the touch. A problem can also occur with old paint that has not been properly deglossed, or with plastic where a suitable plastic primer was not used.

Before painting, the surface should be evenly matte, clean, dry and free of dust. If glossy and matte areas alternate on the surface, the preparation may be uneven.

How to prepare the surface correctly for painting

• remove dirt, grease, wax, silicone and residues of cleaning products,
• sand the surface evenly with a suitable grit depending on the material and the next layer,
• do not sand the surface unnecessarily too smooth or polished,
• remove sanding dust thoroughly after sanding,
• degrease the surface with a suitable degreaser,
• let the surface dry completely before painting,
• use plastic primer on clean untreated plastic,
• on an unknown surface, first test compatibility on a small area.

Why is it not enough to paint over glossy old paint?

Old glossy paint may appear solid and clean, but the new layer may not bond to it properly. A glossy surface is too closed and too smooth for the next layer.

If old paint is to be used as a base, it must be well-adhered, degreased and evenly deglossed. If the old paint is already peeling, cracking or soft, it is not a suitable base for further painting.

What role does degreasing play?

Even a well-sanded surface can fail if grease, silicone, wax, polishing residues or cleaning products remain on it. These contaminants can create a separating layer between the base and the new paint.

Degreasing must therefore be done carefully before painting. The surface should be clean, dry and free of degreaser residues. After degreasing, avoid touching the painted areas with bare hands if possible.

Why can plastic be a problem?

Clean untreated plastic often has poorer adhesion than metal or old paint. For this type of plastic, a suitable plastic primer is usually needed to help create adhesion between the plastic and the following layers.

If paint is applied directly to clean plastic without proper preparation and without a suitable primer, it may later peel, crack or lift in sheets. With plastics, thorough degreasing is also important because release agent residues from production may remain on the surface.

How do temperature and flash-off affect adhesion?

Low temperature, high humidity or poor ventilation can negatively affect drying and curing of individual layers. If the next layer is applied too soon or over a base that has not flashed off enough, solvents can become trapped in the paint system.

This can later cause softening, marks, blisters, poor adhesion or peeling. That is why it is important to use suitable temperature, thin layers and enough flash-off time between each step.

What to do if the paint is already peeling

If the paint starts peeling, simply spraying over the problem is usually not enough. The new layer would be applied over an unstable base, and the same problem could return.

If the clear coat peels together with the colour layer, it is a sign that the problem is not only in the final layer. Adhesion has failed in one of the lower layers, often already between the first primer and the original surface. In this case, the whole paint system must be repaired from the point where adhesion actually failed.

Peeling or poorly adhering layers must be removed down to a stable surface. The surface must then be correctly sanded, cleaned, degreased and the whole paint system rebuilt using the correct process.

How to prevent the problem

• do not paint over a glossy, greasy, dusty or wet surface,
• always roughen the surface evenly with suitable sanding,
• avoid both insufficient sanding and polishing the surface too smooth,
• use the correct base or primer for the material and paint type,
• use plastic primer on clean untreated plastic,
• keep a suitable temperature during application,
• apply thinner layers rather than heavy layers,
• allow each layer to flash off correctly,
• do not put stress on fresh paint too early.

Summary

• Paint most often peels because of poor surface preparation.
• The surface must be clean, dry, degreased and correctly roughened.
• An insufficiently sanded surface can cause poor adhesion.
• A surface sanded too smooth or polished can also cause problems.
• Paint needs a suitable mechanical surface to bond to.
• If clear coat peels together with the colour layer, the problem often started in the lower layers.
• Peeling of the whole paint system usually points to poor surface preparation or a missing correct primer.
• Simply reapplying the final layer will not solve a deeper adhesion problem.
• Glossy old paint must be deglossed before further painting.
• Clean untreated plastic usually requires plastic primer.
• If paint is already peeling, poorly adhering layers must be removed down to a stable surface.