Usually, masking materials don't last long in a blast cabinet. Especially, such a tiny piece as you'd need to use to protect a dot. I'd be more concerned about the media and any contaminants it may contain. If the media you're going to use has been used before, it may contain microscopic particles of rust. The problem with glass blasting is that it is more of a peening action than a cutting action. So, while the glass blasting may provide a uniform finish, it can also embed tiny particles of rust (iron oxide) under the blasted finish that immediately or in time may continue to rust. Nasty nasty, looks bad to have rust streaks on a stainless, aluminum or carbon steel blued finish. My understanding is that S&W uses glass blasted finish. But, they use dedicated blast cabinets. Colt, on the other hand uses aluminum oxide as a blast medium. At least on their 1911 style slide tops. Aluminum oxide is a much more aggressive media than glass beads. Aluminum oxide uses a cutting action as opposed to a peening action and therefore is much more unlikely to embed contaminants than glass beads. Aluminum oxide can be purchased in different grits. 100 grit is probably the most common. That is the grit Cerakote recommends be used before application of the coating. For other applications a finer grit can be selected for a softer finish. I would make sure any media you may use is clean and adjust the air pressure to match the remainder or original finish.
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