The tin and solder will also be fine if it tarnishes, as long as there is no corrosion from actual moisture on the board. Only if there is moisture you need to coat the board, even if the circuit is not sensitive to leakage currents it would cause electrolytic corrosion which can eat circuit board traces in a short amount of time.
Apart from the whiskers there is another funny thing that happens with tin at cold temperatures, it gets tin pest and turns into a grey powdery substance. Cody Reeder made a nice video about it (Cody's Lab on Youtube). Again this is a mere scientific curiosity and won't affect your circuit, even outdoors.
I missed the glass body diodes comment the first time around, but it is very valid, also for surface mount glass body diodes. They have a tiny expansion gap on one end, between the glass and metal terminal, which you must not fill with coating or potting compound or they can malfunction after thermal cycling.
I agree that polyurethane is a very good choice, you would want some kind of laquer since polish is often extremely dilute stuff, designed not to build up but to leave just a minimal coating that fills tiny scratches. You want whatever would be used to initially coat the wood, rather than the product meant for periodic maintenance and cleaning. Unlike Harvey I would not hesitate to use Acrylic in place of Polyurethane, I have used it in the past (even regular water based acrylic wood paint) with no problems. Some commercial PCB coatings are also acrylic.
ST