sand use varnish do like 3 coats with light sand in between coats paint and then use a spray polyurethane coating. it protects the paint and gives it a glossy look. this is optional, but i recommend it.
You don't have to use marine specific product. You can get a clear out door deck sealer from the hardware store for half the price and it will work fine.Just make sure you put on at least 3 coats.