Paint Defects Guide: Orange Peel, Cratering, Runs and How to Fix Them

Most paint defects are caused not by the material but by small deviations in surface preparation, mixing, ambient conditions and spray gun technique. Diagnosing a defect correctly both shortens the repair time and prevents the same mistake from being repeated. In this guide we cover the nine defects most commonly seen in body shops under four headings: appearance, causes, prevention and repair.

Where should diagnosis begin?

• Inspect the defect under strong light coming from the side, from different angles.
• Determine which layer the defect is in: is it in the clear coat or the basecoat, or does it come from the primer and filler?
• Note the temperature and humidity on the day of application, the thinner and hardener used, and the mixing ratios.
• Before starting any repair, make sure all coats have completed the cure specified in the product's Technical Data Sheet (TDS).

Tip: If the source of a defect is unclear, spray the same mix onto a clean spray-out card as well. If the defect also appears on the card, the problem most likely lies in the mix, the compressed air or the ambient conditions; if it does not, examine the surface preparation of the part.

Defects related to flow and film thickness

Orange peel

• Appearance: The surface has a wavy texture resembling orange peel; reflections are blurred and broken.
• Causes: High viscosity, a thinner that is too fast for the ambient temperature, low spray pressure, excessive gun distance, dry and thin coats.
• Prevention: Adjust viscosity only with the thinner specified for the system, for example ZN-510 Acrylic Thinner, at the ratio given in the TDS; set pressure and distance according to the spray gun manufacturer's recommendations, and apply wet, even coats.
• Repair: Once fully cured, wet sand with P1500–P2000, refine with P3000 and polish. In pronounced cases, sand and apply fresh clear coat.

Runs and sagging

• Appearance: Paint build-up in the form of drips, curtains or waves, especially on vertical surfaces.
• Causes: A heavy coat applied in one go, holding the gun too close to the surface or moving it too slowly, over-thinning, not observing the flash-off time between coats, cold material and surroundings.
• Prevention: Work at a consistent distance and speed, observe the flash-off time between coats given in the TDS, and bring the material and the part up to room temperature.
• Repair: Once fully cured, level the run with the surrounding surface using a hard sanding block and P1000–P1500, or with a run blade; finish with P2000–P3000 and polishing.

Defects caused by surface contamination

Cratering (fisheyes)

• Appearance: Round depressions where the paint has been pushed away from a single point, often with a small dot in the centre.
• Causes: Silicone, polish, grease or fingerprints left on the surface; silicone-containing sprays used in the workshop; oil and water in the compressed air.
• Prevention: Clean before and after sanding with ZN-530 Surface Cleaning Thinner using the two-cloth method; keep silicone-containing products away from the paint area; keep up with the maintenance of the oil and water separators on the air line.
• Repair: If you notice cratering during application, stop spraying; do not try to cover the craters with a heavy coat. Once the paint has dried, sand until the craters have completely disappeared, clean again and repeat the application.

Dust and dirt

• Appearance: Raised specks, fibres or dust particles on the surface of the film.
• Causes: Inadequate surface cleaning, dirty booth filters, work clothing that sheds lint, unstrained paint, particles coming off masking materials.
• Prevention: Change booth filters on time, wear lint-free overalls, strain the paint and use a tack cloth immediately before application; clean door shuts and the insides of the wheels beforehand.
• Repair: After curing, sand only the area of the particle with P1500–P2000, refine with P3000 and polish.

Defects related to solvent and drying

Pinholes

• Appearance: Small, open, deep pinpoint-sized holes in the surface of the film.
• Causes: Air trapped while mixing the filler, a filler surface with unsealed pores, excessive film thickness, baking before the flash-off time is complete.
• Prevention: Mix the filler by folding rather than whipping, and apply it in thin, firmly pressed layers; seal pores with a finishing filler; keep the film thickness within the TDS limits.
• Repair: Sand until the holes have disappeared completely, seal the pores with a thin finishing filler or primer, and reapply the layers.

Solvent popping (solvent boil)

• Appearance: Small bubbles, or tiny crater-like marks left by burst bubbles, mostly on heavily applied edges and corners.
• Causes: Solvent underneath cannot escape because the surface has closed up too early: heavy coats, insufficient flash-off between coats, a thinner that is too fast for the temperature, starting to bake too early or an excessively high oven temperature.
• Prevention: Observe the flash-off times in the TDS between coats and before baking, check the film thickness, and choose a thinner speed that suits the conditions.
• Repair: Sand until the bubbles have gone completely and reapply; in very superficial cases, sanding and polishing may be sufficient.

Defects from underlying layers and appearance defects

Sanding marks showing through

• Appearance: Sanding scratches visible beneath the basecoat or clear coat, sometimes appearing hours or even days after application.
• Causes: Sanding too coarsely before the topcoat, skipped grit steps, sanding primer that has not fully cured, topcoat solvents swelling the edges of scratches.
• Prevention: Follow the grit sequence step by step, dry the primer for as long as the TDS specifies, do the final sanding before basecoat at the fineness recommended for the system, and use a guide coat.
• Repair: Sand until the marks disappear; if necessary, apply primer and redo the basecoat and clear coat.

Dulling and loss of gloss

• Appearance: The clear coat loses its gloss shortly after application or a few days later; the surface looks hazy.
• Causes: Incorrect hardener ratio, hardener contaminated by moisture, curing at low temperature and high humidity, a thin clear coat film, too much thinner, polishing too early.
• Prevention: Prepare the mix at the ratios given in the TDS; for example, ZN-330 2K Acrylic HS Clear Coat should be used with ZN-440 2K HS Hardener, as specified for the system. Close the hardener tin tightly after every use.
• Repair: If the problem is superficial, gloss can be restored by polishing once the clear coat has fully cured; if the film is too thin or the cure is defective, sand and apply fresh clear coat.

Colour mismatch and mottling

• Appearance: A difference in shade from the adjacent panel, or light and dark patches and stripes in metallic and pearlescent colours.
• Causes: No spray-out card, inconsistent gun distance and overlap, over-wet coats, not observing the flash-off time between coats.
• Prevention: Prepare a spray-out card before application, apply even coats with approximately 50% overlap, do not skip the effect coat on metallics, and blend into the adjacent panel when necessary.
• Repair: If the clear coat has not yet been applied, an additional corrective coat of basecoat can be applied; if it has, sand and redo the basecoat and clear coat as a blend.

Quick reference table

Summary

Most paint defects can be prevented with a clean surface, a correct mix, a thinner suited to the conditions and a patient drying process. Always start a repair by identifying the source of the defect, and use the product's TDS as the reference for all times and ratios. If you come across a defect you cannot resolve, get in touch with our technical team.