More than anything a clear coating will protect the painting and make it easier to keep clean. Myself I've quit using oil based varnishes. An oil based varnish yellows with age changing the color of the painting. Blue skies have a green tint to them and white paint starts looking cream colored. The finish needs to be acrylic to remain clear and the easiest product to find is Polycrylic. Also being a furniture refinisher, I just put polycrylic in a sprayer and spray my paintings but it can also be brushed. With oil based paintings a person needs to wait six months to a year before clear coating depending on the thickness of the paint. With acrylic in reasonably dry weather it could be ready in a week. Damp weather makes any paint dry slower.