No matter how well you control your powder coating process, you will undoubtedly have defects that need to be reworked. Here are a few simple things that you should know before you make this defect worse and end up scrapping the part.
Before you can begin to figure how to fix a defect, you need to know what you are dealing with. Proper identification is priority number one. First, if the defect is so severe that it cannot be corrected for a cost that is less than the scrap value of the part, you should toss it out without reservation. Next, if the defect is a substrate metal flaw or fabrication mistake (i.e., metal scratch, dent, missing weld, etc.), repainting it is unwise, as it will not correct the problem. Send it back to fabrication for resolution. Finally, if the defect is related to the coating process (i.e., light paint, dirt, fisheye, crater, etc.), then proceed to the next steps: surface preparation and recoating.
Surface preparation procedures need to be matched to the specific coating defect. You have several options:
Light paint defects require no surface prep, except for recoatability issues with the original powder coating. Check with your powder supplier to find out the recoatability of the formula you are using, as some formulas contain wax, Teflon, or other slip agents that make recoating difficult. Additionally, some formulas do not have good intercoat adhesion properties, requiring surface sanding before repainting the part.
Dirt, spits, seeds, bumps, craters, fisheyes, sags, mars, etc. all require spot sanding to remove the surface defect and smooth the surrounding area to a feathered edge before repainting. Start with 120 grit sandpaper and work towards finer grades until the surface is smooth to the touch.
Small localized surface defects can be removed using wet sanding techniques with very fine (1000 to 2000 grit) sandpaper. In this case, keeping the repair area as small as possible is best. Finish off the repair using automotive rubbing/polishing compound and wax using a variable speed polisher and wool bonnet. This method is preferable to recoating the entire part. Of course, if you are using rough or textured coatings this approach will not work and you will have to recoat the part to repair the defect.
Defects that are too large for spot repair require complete stripping to remove the original coating. Media blasting (plastic, glass bead, oxide, etc.) can be a fast way to remove the coating. Chemical stripping (hot or cold) can also be effective. However, most people use thermal stripping (back-off, burn-off, molten salt, etc.), as it is quick and cost effective.
After surface preparation, the next step is to recoat the part. Here are your options:
Recoat the parts on the line. When recoating localized repaired areas, turn off the active washer stages (wash, conversion, and seal) to prevent spotting, and use the rinse stages to remove the sanding dust and prepare the part for recoating. However, if the part has been completely stripped, run it as you would a bare part with all washer stages operating. Be sure to apply the new coating to full specified film thickness to achieve the look and feel desired.
Small surface defects and light paint areas can be repaired using liquid touch-up paint, alone or after light sanding. This is the best approach if the defective area is not critical to the functionality of the part, since most liquid touch-up paints do not perform as well as the original powder coating. Liquid touch-up paitns can be packaged in a variety of devices that make their use very convenient (i.e., pens, “white-out” bottles, spray cans, bulk containers, etc.).
In the event that you encounter a defect after powder coating your part, be sure to identify the defect, prep the surface and then pick a method to recoat the pat. Sometimes, no matter how well you control your powder coating process, a defect can occur. Knowing these simple steps can keep you from making the defect worse and having to scrap the part.
Nick Liberto, P.E.,