Varnish, synthetic resins and other clear film forming finishes provide an attractive finish for wood since they allow the natural wood color and grain to show through. Unfortunately, the durability of these finishes on wood under the action of sunlight and moisture is limited.
Regardless of the number of coats applied, the film will begin to crack and peel, and the finish will have to be completely removed by sanding or with a varnish remover before a new coat is added. Alternate finishes such as semi transparent stains and water repellent preservatives will give a longer service life and are easier to refinish.
Exterior marine or spar varnishes may be used with some success on exterior doors and other areas if adequate protection from the weather is provided. Never wait longer than a year before re-coating an exterior varnish.
The power of the sun will damage the bond of the finish where it meets the wood, causing it to crack. Adding UV filters may help, but tend to darken the finish, often to unacceptable levels.