A.For previously unfinished wood, a urethane finish would be the best option. Urethanes hold up well to ultraviolet light. However, urethanes tend to be brittle and can crack if the surface is subject to impact.
For wood that has already been coated with a clear finish, your best bet is an alkyd-based varnish, especially if you’re not sure what the original finish is. Alkyds are compatible with a wide range of finishes, whereas the solvents in urethane may cause some clear finishes to bubble. A very light coat of the alkyd-based varnish might yellow...
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