The phrase, which means to repeat the same task over and voer again, comes from the original practice of painting the Forth Bridge. A team of painters would commence painting at one end of the Bridge, and work their way to the other end, which took i believe a couple of years, and by that time, it was time to start the process over, so a team was always painting the Forth Bridge. I think the practice is still in use today, but with modern paints, it may be that the paint lasts longer.