I am trying to diagnose the spray arms not turning. As a first step I am trying to remove the lower spray arm by turning the flat circular piece at the center of the arm. However I've been unable to get it to budge. I've tried applying vinegar around the edges with a toothbrush to loosen it with no improvement. Are there any special tools or other lubricants I might try?

Will I be able to remove the base without first removing the spray arm?

Thanks in advance for your help.