once you receive back your block from the machine shop after reboring ,it contains a lot of small metal particles all over and in the oil line so it has to be flushed/washed with high pressure water to clear all the oil gallery and block from the metal particles.but as blocks are made from cast iron so a thin layer of rust comes on it immidiately after washing.So high pressure air is used to clear all the oil gallery from water that may be present there after washing and then a thin layer of engine oil is applied on the block and small amout is poured into the oil gallery holes as well .It comes out something like this...