Teak Oil vs Spar Varnish
There are 2 main types of clear outdoor finish; teak oil (penetrating oil), and spar varnish:
Teak Oil (Penetrating Oil)
Penetrating oils, often called teak oil (outdoor finish) or danish oil (indoor finish), soak into the wood and don't build up on the surface. They may contain linseed, rosewood, or tung oil, and an assortment of other ingredients. Teak oil isn't made from the teak tree, it is so named because it is often used on teak wood.
Most penetrating oils are very easy to apply. Wipe the oil on with a brush or rag, wait a few minutes for it to soak into the wood, then wipe off the excess. Penetrating oils will never chip, crack or peel, since the finish is in the wood. If the wood is fully exposed to the sun and rain year-round and you want to keep the wood looking like new, most penetrating oils need to be reapplied 1 to 2 times a year. If the wood is protected from the elements by shade, such as a porch, you may only need to reapply the oil once every few years. Generally speaking, as the amount of pigment is increased in a penetrating oil, the length of time increases before you need to reapply it.
An excellent penetrating oil is Watco Teak Oil. It contains boiled linseed oil, Ultraviolet (UV) light inhibitors for sun protection, and a mildew inhibitor. Linseed oil is a natural finish made from flax seeds, a common ingredient of paint and finishes for centuries. It works well on any type of wood, including dense woods like Teak, Mahogany, or Rosewood. If you get a scratch in the finish, it can be easily touched up by simply re-oiling the area.
Marine spar varnish and exterior polyurethane finishes usually soak into the wood a little bit, but mainly build up on the surface. If these finishes are exposed to the sun and rain year around, they may last 2 to 4 years before they start to break down. Purchasing one with maximum UV (Ultra Violet Light) protection is important. If the wood is fully exposed to the elements you should lightly sand and re-coat every 1 to 2 years. If you don't re-coat it and the finish cracks or chips, you will have to strip the old finish off and start again.
An excellent spar varnish is McCloskey Man O' War Marine Spar Varnish. It contains tung oil and polyurethanes, which combine to form long chain molecules, called polymers. Polymers are more flexible than regular polyurethanes, so they resist cracking during temperature fluctuations (day to night, etc.). As a result, the finish lasts longer.
Always experiment first in an inconspicuous place for color, shine, etc.. When you are ready to finish the entire piece, coat all sides of the wood, including the underside of furniture. If you don't, the unfinished side of the wood may dry out faster than the finished side, resulting in warping of the wood. If the wood warps, the finished side will become stressed, the finish may crack and chip off, and fail sooner than expected. Coat the bottom of the legs to prevent water absorption, and you will add many extra years to the life of your wood.
Read and follow the directions and safety precautions on the can of any product you choose. Dispose of used rags according to the directions on the can.