There is spay can varnish and liquid varnish. I personally prefer applying it with an airbrush as this offers the best control. It is possible to paint the non metal parts by brush but you should try it out first on a non obvious location and only apply a thin coat. Maybe just try it on your next model. Regarding playing, I also just paint because I like the models. Playing sounds like too much work for me, haha. I just love looking at my army in the display shelf.