Can this work as a waterproofing method? Im making my garden automated with temperature, humidity and soil temperature sensors. I want to be able to connect my Arduino Uno to a solar panel and keep it outside at all times. I dont want to have to remove it to water the plants. Could I cover it in clear nail polish to water proof it?
Nail polish would work in general as a fair conformal coating. However, conformal coatings are not generally intended as waterproofing, but moisture or humidity resistance.

If you product is going to be regularly submerged or deluged with water, you are better off using a waterproof enclosure with a good watertight gland around the cable entrance.