What are the main challenges in maintaining and coating metal fabrication assets?
Painting metal is a challenging business and there are any number of things that can lead to a coating failure – resulting in poor aesthetics and even material performance issues.
This post looks at some of the major problems in coating metal fabrication and how to ensure your project isn’t affected by them.
Understanding the different paint defects that can affect metal fabrication assets.
Description: A term that is generally applied to a sprayed material that misses the object being coated or is created by bounce back off the surface of the object being painted.
When airborne atomised particles settle on a surface that is already painted, a dry profile is created, this is also often described as overspray.
This effect can be reduced by improving the transfer efficiency of the material from spray gun to object, using the correct spray equipment, gun set-up and spraying pressures.
A slower thinner or retarder solvent can also be used to keep the painted surface “open” or wetter for longer, which will help absorb any overspray or spray mist into the paint film.
Description: Small blisters with a microscopic hole in the centre. Pinholes can have several different causes but are relatively easy to avoid.
It can be caused by
- Film thickness that are too high, ensure the correct number of coats are applied with the specified flash-off between coats.
- Using a solvent that evaporates too quickly.
- Metal temperatures that are too high when the paint is applied.
- Flash-off times being too short before force drying.
Rectification often requires the surface to be fully sanded and the coating re-applied.
Bleeding or staining:
Description: Bleeding produces a discolouration of the top coat, usually resulting in a redder or yellower colour shade. White or paler colours are worst affected.
- Soluble pigments or colourants from the original paintwork being dissolved by the solvent contained in the refinish materials.
- Bleeding can also occur when excessive amounts of peroxide curing agent is used in polyester body fillers, which are again dissolved by solvents usually in the primer coat.
- Failure to remove bitumen or tar residues from the existing paintwork.
Rectification will largely depend on the severity of the staining but will generally require a 2k epoxy primer or a purpose made stain blocker to effect a permanent cure.
Wrinkling / lifting:
Description: Wrinkling or lifting means that an uneven, sometimes a furrow like pattern develops on the surface of the paint film during the curing process. This indicates that the surface of the coating is drying faster than the underlying layers.
This defect only happens with a synthetic type paint system.
- Synthetic paint being applied too thickly.
- Unfavourable drying conditions i.e. ambient temperature very high drying the surface of the finish.
- Applying a second coat outside of the recommended overcoating schedule.
- Wrong thinner being used.
Rectification: If the wrinkling is minor or localised, allow the surface to through dry completely. The surface will then require sanding back to a sound coating before applying a new finish. If the defect is more severe, the whole of the affected coating will have to be removed before re-coating can be successfully achieved.
Run and sags:
Description: This is a defect that has many causes but will always result in a downwards movement of the coating, forming drips or runs. Sagging is a term used for a run that extends over a significant area and is often called a curtain.
- Paint thickness applied more than the manufacturer’s specification.
- Excessive thinner added to the paint.
- Metal surface being too cold.
- Incorrect spraying technique used.
- Wrong size spray tip.
- Gun held too close to the surface.
- Incorrect overlapping technique.
Rectification: If the run is caught early it could be brushed out while the paint is still wet. Although this may not achieve a visually acceptable result, it will make the final rectification easier to achieve.
Allow the run to through harden and sand the affected areas before repainting.
Small runs may be flatted out and buffed, but this technique will entirely depend on the type of paint system used. This process would be unsuitable for single pack paint systems.
Orange peel finish:
Description: Orange peel is a visually textured imperfection in the paintwork. This can range in appearance from slight to totally unacceptable.
The level of orange peel will also determine the method of rectification required.
- Improper gun technique.
- Wrong spray tip.
- Incorrect paint viscosity.
- Wrong thinner used.
- Atomising air pressure too low.
- Paint applied too thickly.
Rectification: Regardless of the severity of orange peel, the only solution is to flat the paint surface.
What happens next depends on:
- The level of finish required.
- The type of paint system used (1K or 2K).
- Car refinish systems would generally be buffed and polished to achieve the finish and gloss level required.
- IK paint systems would require flatting using a fine grade abrasive wet-or-dry paper and recoated.
Description: Light, whitish circular spots, which can either swell or indent the surface of the coating.
Depending on the exposure time to moisture and the freshness of the finish, the severity can range from mild, (possible recovery when dry) to severe, requiring re-coating of the finish.
Description: A milky white haze or mist formed on the surface of the paint film.
- Spraying during cold, wet or humid weather.
- Solvent evaporating too fast or poor quality thinner.
- Spraying pressure too high.
- Blowing the surface with compressed air to dry the surface.
- Inadequate heating or poor air movement.
Rectification: This will depend on the severity of the blooming and may only require a wipe with a cloth after the paint has fully hardened. Often the blooming is permanent and will require the coating to be flatted and re-painted.
It may also be noted that when painting in these conditions, this effect has also happened to the primer coat, which although may not be immediately obvious, the primer may contain moisture, which could develop micro-blistering later.
Fisheye / Silicone contamination:
Description: Fisheyes are a coating defect that is characterised by crater like openings in the finish. Fisheyes are also known as: silicone contamination, saucering, pits, craters and cissing.
- Improper or insufficient cleaning or preparation.
- Old finish or repairs.
- Contaminated air supply.
- Oil, wax, grease or silicon valeting sprays.
- Silicon containing polishes.
Rectification: Minor interruptions can be corrected during the application by using light mist coats to cover the affected areas.
More severe contamination may require the surface to be flattened and re-painted.
Pronto Paints have been working in metal fabrication for over 30 years and have an exhaustively researched range of metal fabrication coatings. Click here to check out our technical specifications.
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