The Benefits of Spot Repair
How can you avoid the time and expense of running parts with minor damage back through your paint line? PPG’s Michael Kowalski recommends spot repair.
Q. We sometimes have to repair parts after they’re coated because of isolated scratches, dings and other damage that occurs during packaging and shipping. How can we avoid the time and expense of running these parts back through our line to repair them?
A. Finding a defect on an otherwise perfectly coated part is extremely frustrating, but it’s part of the business. Although repainting the entire part is sometimes the only option, many damages can be efficiently fixed through a dedicated spot-repair process, which can reduce rework, save money and minimize disruptions to your coatings operation.
It’s important to analyze your shop’s readiness before implementing a spot-repair process, however. The main considerations are:
- Space. Do you have an area in which the damaged part/unit can be safely sprayed? It could be as small as a manual booth or as large as a “spoven” (spray booth/oven).
- Time. How much time and labor will you save by repairing damaged parts in an offline area compared to completely resanding and repainting?
- Part size. Large parts are typically easier and more cost-effective to repair than small ones.
- Class of parts. Does the performance/quality specification (Class A, B or C appearance grade) require that the entire part be refinished to meet customer requirements?
- Gloss. Damaged parts with high-gloss finishes are easier to repair.
Once your shop is ready, you will need to equip it with:
- Gloves, safety glasses and respirators*
- Nib sander
- Dual-action sander
- Small precision/finesse gravity-feed spray gun
- Various grit sandpaper
- Waffle polishing pad
- Rubbing compound
- Metal surface conditioner
- Blending solvent
- Deionized water
- Machine polish and an ultrafine machine polish
- High-performance polishing cloths
- Lint-free cloths
- Isopropyl alcohol (IPA)
A robust spot-repair process is agile enough to address a range of defects. The three most common levels of repair, and instructions to execute them, are provided here:
Minor defects (such as repairing a nib or piece of dirt in the paint film). Sand the repair area with the appropriate-grit paper until the defect is flat, then clean the surface completely of dust using a lint-free cloth and IPA. Apply a rubbing compound to remove any fine scratches left from sanding, then apply a polishing compound with a finer grit than the rubbing compound.
- For less aggressive sanding, lightly spray the sandpaper and/or defect with water before starting.
- When sanding, do not break through the topcoat to the primer film.
- Use a higher-grit paper when stepping-out the sanded area.
- Polish the sanded area until the sanding scratches are gone.
Moderate defects (such as reapplying topcoat over an intact primer layer). Follow the same sanding and cleaning procedure and precautions used to repair minor defects, then apply multiple coats of the production topcoat. Step-out each application to create a smooth transition from the repair area to the initial coat. To address the overspray between the wet paint and the cured paint film, spray a blending solvent past the overspray area using a small precision/finesse gravity-feed spray gun. This must be done within minutes of applying the topcoat. Spraying too much blending solvent will cause the blender to run. If this happens, wipe off the entire repair area, and respray the topcoat and the blending solvent. After curing via air-drying or infrared light, the part will require little or no polishing.
- During sanding, keep the sander flat to the surface to avoid making gouges.
- Clean the sanding area with IPA to remove sanding dust.
- Minimize the overspray of the topcoat so it lands on a masked area of the part or in the wet area of the repair.
- Polish only when the topcoat is completely dry.
Severe defects (such as removing paint and primer down to the metal substrate). Sand several inches in diameter around the defect, penetrating down to the substrate. Leave one to two inches of primer exposed on all sides of the area. If the exposed substrate is metal, spray the area with a metal surface conditioner and let it set for two minutes, then spray the treated surface with deionized water to offset the resulting chemical reaction (which forms the protective coating). Dry with a lint-free cloth. Apply several coats of primer using a small precision/finesse gravity-feed spray gun. Allow time for drying between applications and stepping-out each coat. Sand the overspray of the primer and wipe away any sanding dust with IPA before applying the topcoat. For the topcoat application, step-out each coat beyond the sand marks to avoid a “bull’s-eye” effect. As soon as possible, spray the blending solvent past the overspray area to create a smooth transition between the wet and dry areas. Cure and apply polish, if necessary, when the topcoat is completely dry.
- Do not spray the blender during the primer application; cure the primer overspray and sand off the primer to eliminate overspray.
- If the sandpaper gums up, the primer is not sufficiently dry.
Along with providing high-quality parts to customers, paint shops are continuously seeking ways to improve efficiencies and be more cost-effective. Setting up and maintaining a spot-repair area achieves those objectives while providing arguably the biggest advantage: satisfied customers.
Michael Kowalski is a product manager of industrial coatings at PPG Industries. Visit ppgindustrialcoatings.com.
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