What is the cause of the bubbles on the steel flexible expansion joint?

The pipeline is used for underwater passage. It is generally very useful and advocated when removing underground sewage, or draining floods.
Some careful customers of steel flexible expanders (also known as steel expansion joints) will notice that some tiny bubbles sometimes occur on the steel flexible expanders. In fact, this is from the process of spraying. Before the steel flexible expansion joint leaves the factory, it has to be sprayed to improve the moisture-proof property of the joint on the one hand, and to keep the joint from being leached by the weak acidic liquid on the other hand. However, in the case of spraying, sometimes also due to internal and external causes, resulting in the occurrence of vapor bubbles, meaning that in the case of spraying into the gas, so it must be sprayed carefully in the case of spraying, so as to reduce the small bubbles caused.