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SwedenHow to Fix Paint Runs and Sags
Paint runs and sags are most often caused by applying too much material, spraying too close to the surface or moving the spray can or gun too slowly. Temperature, thinning ratio, spray-gun settings and insufficient flash-off time between coats can also affect the result.
The good news is that a run in clear coat can usually be levelled by sanding and polishing once the coating has fully cured. The most important points are not to rush the repair and not to sand through into the colour or effect layer underneath.
Why paint runs
A paint run forms when there is more liquid material on the surface than the coating can hold before it begins to flash off. The most common causes include:
- applying a coat that is too heavy or too wet,
- moving the spray can or gun too slowly,
- spraying too close to the surface,
- repeatedly spraying the same area,
- allowing too little flash-off time between coats,
- over-thinning the material or using an unsuitable thinner,
- working at a low temperature, which slows solvent evaporation,
- incorrect spray-gun settings or excessive material flow,
- holding the spray can at an unsuitable angle.
When using a spray can, keep it upright with the valve and nozzle facing upwards, usually at a distance of approximately 20–30 cm from the surface. Always follow the specific spraying distance and flash-off times stated for the product being used.
What to do when the paint has just started to run
Do not touch or wipe the run while the coating is drying, and do not apply more material to the affected area. Disturbing the coating while it is still soft can tear the surface, create ripples or damage the colour layer underneath.
The safest solution is to allow the coating to cure completely and repair it afterwards. The curing time depends on the type of coating, temperature, film thickness and drying method. Follow the technical data sheet of the product used.
Before starting: assess the type and extent of the defect
- Small run only in clear coat: it can usually be levelled and polished.
- Large or raised run: it may be helpful to carefully reduce the highest point before sanding.
- Run on an edge or body line: the risk of sanding through is very high, so work with extreme care.
- Sanding through into the colour or effect layer: polishing alone will no longer be sufficient and the damaged layer will need to be restored.
- Run in an effect paint: localised sanding may alter the shade or effect, so the entire visible section often needs to be refinished.
I. Repair by sanding and polishing
This method is suitable when the run is only in the clear coat and there is still sufficient coating thickness underneath it.
1) Reduce the highest point of the run
Once the coating has fully cured, a strongly raised section can be carefully reduced with a suitable paint-run scraper, nib file or razor blade with protected edges. The tool must only contact the highest point of the defect and must not touch the surrounding clear coat.
2) Wet sanding with a firm sanding block
Wet-sand the affected area using a small, firm sanding block. Do not sand only with your fingers, as they will follow the shape of the run and remove unnecessary clear coat from the surrounding surface.
Depending on the height of the defect, you can begin with P1500 wet sanding paper. Once the run is almost level, continue with finer abrasives:
- P1500 – carefully levels the raised defect,
- P2000 – removes the sanding marks left by P1500,
- P2500 to P3000 – prepares the surface for easier polishing.
Dry the surface regularly and inspect it against the light. As soon as the run is level with the surrounding area, stop using the coarser abrasive.
3) Polishing
After levelling the run and refining the sanding marks, polish the surface with a suitable polishing compound. Begin with a corrective compound and, where necessary, finish with a finer polishing product.
Polish edges, body lines and sharp transitions using minimal pressure. The coating is usually thinner in these areas and can easily be sanded through or overheated with a polishing machine.
II. Advanced repair using a guide layer of finishing filler
For a more pronounced run, a very thin layer of fine finishing filler can be applied after the clear coat has fully cured. The filler is spread over the run and its immediate surrounding area.
During sanding, it serves two purposes:
- it helps protect the level surrounding clear coat from unnecessary material removal,
- it acts as a guide layer, making it easier to see when the highest point of the run has become level with the surrounding surface.
Procedure
- Allow the clear coat to cure completely.
- Apply only a very thin layer of fine finishing filler over the run and its immediate surrounding area.
- After the filler has cured, begin sanding carefully with a small, firm sanding block.
- Once the filler has disappeared from the highest point of the run and the surface is level, stop sanding with the coarser abrasive.
- Remove all remaining traces of filler and continue with finer abrasives, for example P1500 → P2000 → P3000.
- Polish the surface afterwards.
III. Repair by sanding and applying a new clear coat
Choose this method if the run is too high, the clear-coat film is too thin, the surface cannot be safely polished or sanding has exposed the layer underneath.
Procedure
- Allow the coating to cure completely.
- Level the paint run.
- Evenly matt the entire panel or the complete visible section.
- Clean and degrease the surface thoroughly, then wipe it with an antistatic tack cloth.
- Apply a new clear coat according to the technical data sheet of the selected product.
When preparing the surface for recoating, follow the abrasive grade recommended by the clear-coat manufacturer. A surface that is too smooth may reduce adhesion, while scratches that are too coarse may remain visible after recoating.
Suitable products can be found in our clear coats category.
What to do if you sand through into the colour layer
If you sand through the clear coat and damage the colour or effect layer, the area usually cannot be repaired simply by applying more clear coat. The damaged layer may show a different shade, a visible patch or a different orientation of the effect pigments.
- With a conventional solid-colour basecoat, the colour layer may be restored depending on the extent of the damage, followed by clear coating the entire panel.
- With Candy, Candy Pearl, pearl, Flip-Flop and other effect paints, we recommend refinishing the complete visible section so that the shade and effect remain consistent.
Common mistakes during repair
- starting the repair before the coating has fully cured,
- sanding with fingers only and without a firm sanding block,
- sanding for too long with a coarse abrasive,
- failing to inspect the surface regularly during the repair,
- using excessive pressure on edges and body lines,
- attempting to repair a run in effect paint in the same way as a run in clear coat,
- applying new clear coat over a surface that has not been sufficiently matted,
- trying to polish out the run without first levelling it.
Quick guide
- Fresh paint run: do not touch it; allow it to cure completely.
- Small run in clear coat: P1500 → P2000 → P3000 → polish.
- Raised paint run: carefully reduce the highest point, then sand with a firm block.
- Guide method: apply a very thin layer of finishing filler over the fully cured run.
- Sanded through into colour: restore the colour or effect layer and apply clear coat again.
- New clear coat: matt the entire panel or complete visible section according to the product TDS.
A properly levelled paint run can often be removed so effectively that, after polishing, the repair is not visible under normal viewing conditions. Work slowly and regularly check how much material you are removing.