Busy powder coaters will agree: silicone plugs are life-savers. They’re the quickest, most fool-proof way to ensure that blast media and powder coating stay out of vulnerable, threaded areas, and they’re reusable to boot.
Like any other tool in your shop, though, silicone plugs require maintenance.
If, like Kaser, you leave your plugs in through powder coating and curing, buildup is inevitable. That plug will eventually turn into a visual record of every powder coating color you’ve used in its lifetime, surrounding the silicone like hardened tree rings.
Using a built-up plug on a fresh part is not ideal. The plug’s old powder is liable to fuse to the part’s loose powder during the curing process, causing minor problems after the part has cooled – when you go to remove the plug, you’re essentially tearing the part’s fresh powder coating off of the plug’s old powder coating. Much like ripping a sheet of paper in half, this creates a raised, jagged, flaky powder border (called a “kerf”) around the masked area’s edge.
Most of the time, this is not a huge problem. Scraping the kerf with your fingernail is typically enough to get rid of it. However, it’s one more detail to deal with before the part leaves your shop. Kerfs look sloppy – best to remove them before the customer picks up the part.
More problematic is when a built-up plug introduces a foreign color onto your part’s surface. I’ve seen black parts wind up with bright green residue around masked openings, which is not a good look. Sometimes, the residue can be gently scraped off with the kerf; sometimes, it can’t.
The more diligent you are about cleaning off your plugs between each project, the less buildup you’ll have, and the longer you can re-use that plug. Some powder coaters even advocate for removing the plug before the part goes in the oven. While this is a great way to avoid a buildup of cured powder on the plug, it’s also a bit dangerous for the fresh coating you just sprayed – one slip of the hand, and you could knock loose powder off of the part before it cures.
Ultimately, though they are reusable, no plug is eternal. Whether because of old age or powder buildup, you will need to replace your silicone masking elements sooner or later. The more proactive you are about doing so, the less likely you are to encounter kerfs.
Should you encounter a kerf, don’t panic: try running your fingernail over it. It will likely flake off as though it never happened.
In summary: if it’s been a while since you looked at your silicone plugs, there’s no time like the present. If you can, chip off the buildup. If you can’t, order new plugs. While a kerf isn’t the end of the world, it is a nuisance that can easily be avoided with proper maintenance.