The line that may develop around the water line area of your spa has two main causes:
Cause 1: Mineral Deposits
This is more common with those that use bore water or other water that is overly hard and it is basically a deposit of minerals left as the water evaporates away.
This deposit is usually white or rust coloured and will be chalky to the touch. This deposit can usually be removed using a strong solution of vinegar. If you have this you need to be extra vigilant with your water hardness.
If you have a large amount of minerals in your water we recommend using a sequestering agent that will grab the minerals and bind them together where they can be caught in the spa filter.
If you have a tiled or concrete spa you will want to use a product suitable for those materials.
Cause 2: Oils from Sunblock, lotions and bodies
This is probably the most common cause of the line around spas. This is a deposit of body oils, lotions or sunblock and is usually dull in appearance and can be slick to the touch.
One way of preventing this is requiring bathers to shower before sitting in your spa but this is not always practical.
Many people choose to use a spa cleaning disc to reduce oils that float on the surface of the water. This simple device will not only reduce the scum line caused by oils but it will also help your filters work more efficiently by grabbing oils before they can plug up the small openings in the spa filter material.
This line caused by oils can be removed by using a small amount of pipe degreaser on a towel.
You will also want to use this degreaser just before emptying the water in your spa as it will remove any of the build up on the internal surfaces and reduce future chemical use with the new water.