My experience with varnishes, urethanes etc has been that:
1. they yellow a lot if they are exposed to sunlight and they do not
yellow evenly. Marine varnish is usually pretty amber to start with due to
large amount of UV inhibitors.
2. the exposed edges of applied clear coats are vulnerable to wear (not
unlike the edges of decals)
3. if you ever are called upon to do repairs, the fresh paint will be much
brighter and a totally different colour than the original under-varnish
paint. I've found that if there is any chance that repairs or touch-ups
needed in the future I always avoid varnish or any protective clear coats.
Also if the clear coats start discolouring, becoming hazy due to wear, or
even peeling off, the owners will come back to you for repairs.
If they do insist on clear coats for protection let them know of the above
possible eventualities so they can't say they 'didn't know'.....
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