Plein Air Painting is Painting Outdoors - On Site. This is one of my favorite activities! I enjoy this like folks enjoy Golf. In fact I often go when my husband is golfing with the guys.
When painting "en Plein Air" I choose to paint with Watercolor. It is easily transported as it dries almost instantly. I either paint on a panel that I create ahead of time, or on a watercolor block which prevents the paper from buckling when wet. The Plein Air Panel consists of watercolor paper mounted on Gator Board. This also prevents buckling and as quick as it dries it can be popped into a frame for a plein air competition.
All of my watercolor paintings are sprayed and sealed with UV Archival Krylon Varnish. This protects them and seals them and allows me to frame with out mat or glass. No different than oil paintings.