Avoid people trying to sell you their product. Listen to the pro's and what they use as they have financial motivation.
Marshall Contracting hit it right. Seal all 6 sides. Use 3 nails per board. Space 1/8th inch apart for drainage. Place wood with the smile facing up. (look at the wood and it has a smile and a frown on the graining.This will prevent cupping.

First, you should turn off the lights you are going to clean and remove any light bulbs. Then take a lemon wedge and cover it in salt. the acidity of the lemon will help loosen any tarnishes and the salts texture will help to scrub off the tarnishes. Afterthat, take a damp cloth(microfiber would be the best choice) to wipe off the salt and lemon juice. Next use a damp sponge anywhere the lemon and salt didnt remove the tarnishes to dampen the area. The next step would be to cover the damp lamp in a powder copper polish. the moisture will help the polish adhear to the light fixture. let the polish sit on the lamp for a few minutesbefore using the damp sponge to wipe it off. The remaining tarnishes should come off with the polish. then just dry off the light with a soft cloth. This little trick should get your light looking new again quickly.