About to make a mold for my popgun need advice

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I have been working on a project for quite some time but had a few questions before I attempt the pricey mold.

1. After primer is clear coat the best for a master before mold making?
2. Any suggestions for mold design for the huge bore barrel 4-5" and 6in deep? I'm thinking 3 piece mold but really have no idea.
3. Any other suggestions for the mold and casting?
4. I will hand turn the mold as the cast sets. Planning on pouring turning then when almost set pouring and turning thin layers at a time.

http://atelierpoupee.blogspot.com/2010/12/mold-making-tutorial.html?m=1 found this amazing article! The plug looks a little ummm plugish lol but the details on the link are priceless!