@Nicholas - it is not so much using a spiral mixer tip as it is the epoxy that you are using in the first place. The current epoxy that Blue Robotics is using on their motors / penetrators works just fine. I mix epoxies all the time and I don’t worry about the things you have mentioned … never had a problem with dust/dirt, moisture or the air bubbles. Air bubbles only happen in the lower viscosity fluids to a point that even start to create or make you think you have a problem.
The motors were tested at pressures much higher than anyone on here will ever go with them … and they didn’t leak. That pretty much proves it in my book.