[orange4boy]

I hate to be the bringer of bad news but varnish and most polyurethanes have notoriously poor UV protection. They are meant to go on thick and with many coats and they all yellow with age even if they have some UV protection.

The automotive clear coat solution is probably the best followed by automotive clear spray cans and then rustoleum spray can clear. Just don't use clear lacquer unless it's automotive acrylic lacquer. Lacquer meant for wood will attack the plastic.

These days you can get very good quality automotive spray can clears.

BTW, you don't need an expensive kit to do this job, just a fine polish, a cloth and elbow grease. Paint polish works as well, paint is basically plastic anyway, although, if you want to spray it after make sure you use a paint ready polish without wax or you will get fish eyes. It's a good idea to clean the lens with mild solvent before spraying in any event. A mix of 25% water to 75% rubbing alcohol (Isopropyl) can be used for plastic.