like painting the Forth Bridge

Etymology
The bridge across the Firth of Forth was in a continuous state of being painted (until finished in 2011, 122 years after its completion): (owing to its large size) as soon as one end was reached, it was time to begin all over again.

Prepositional phrase

 * 1)  Never-ending; said of a task which will never be completed due to the perpetual cycle of having to recommence as soon as one has reached the end, or of efforts that are in vain because of this.