Good instructional video. It never occurred to me to apply the poly finish after the airplane is assembled. I'll have to try that someday.
I always coat the foam board sheets on both sides the day before I begin a build. Coat both sides so there's no warping. Minwax makes a wipe-on poly finish. I prefer the wipe-on poly because it is thinner than the brush-on poly. Since it is thin it goes on faster and gets absorbed faster. Since you use less of it there's weight saved too. Instead of using a brush or rag to apply the Minwax I pour a small amount right on the paper and spread it with a squeegee. A credit card makes a good squeegee. I can cover a whole sheet in less than half the time it takes to brush it on. Not a drop of Minwax gets wasted and there's no brush to clean or dispose of.