like painting the Forth Bridge

Arising from the fact that the bridge across the Firth of Forth was in a continuous state of being painted (until finished in 2011, 122 years after its completion), because (owing to its large size) as soon as one end has been reached, it is time to begin all over again.

like painting the Forth Bridge

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.
   • 2014, Stephanie Owen, ‎J. R. Lewis, Law for the Construction Industry (page viii)

     Where only an overview has been given, I hope that the book will provide a useful springboard and the appropriate legal advice is sought. The revision has been rather like painting the Forth Bridge and whilst every effort has been made to bring the law as up to date as possible, events often overtake one and a keen eye should be kept on the construction journals and newspapers.

• Sisyphean