Vitamin C will remove rust from most anything. I know it removes it from inside my swimming pool.
Get some vitiman C tablets and crush them inside a sock. Wet really good and rub the rust with the wet sock after the vitamin C has absorbed into the sock.
Does he trick and everyone usually has vitamin C around the house.
We had rust on a stamped concrete driveway and were told that floor neutralizer would remove it. They were right but no body told us the colour they apply to the concrete is also made out of iron and all that colour disappeared too