A hollow profile with reinforcement on the inside should contain openings allowing air to escape.
The small difference in specific gravity between steel and zinc means that, when an object is being galvanised, it must fill up with zinc, and the air should be getting out, as quickly as possible. Air pockets are particularly likely with hollow profiles. Even a small amount of enclosed air will make an object float on the zinc bath. As soon as this happens, the flux layer will burn, and galvanisation becomes impossible.
Inserted sections of pipe
With inserted sections of pipe, the joint should be as smooth as possible so that no liquid is left behind.
When a section of pipe is inserted too far, creating an edge inside the pipe, the pickle or the flux liquid will accumulate behind that edge. This has a negative impact on galvanisation quality. When the section of pipe is hung upside down, the liquid will flow away but air pockets will be created, leading to non-galvanised spots.