The 1st thing is to make sure the surface you are going to poly is GLASS SMOOTH - (if it isn't glass smooth you will not get a smooth finish). Then I normally spray the poly on. Your spray setup will determine the pressure and amount of thinning you will need to use. (A tip - when you can spray a VERTICAL piece of glass without any sags, runs or orange peal, you are ready to spray the plane.)
FWIW - Oil base poly WILL start to yellow in about 6 months (or less). Water base poly WILL NOT yellow. A poly finish is fuel RESISTANT, NOT FUEL PROOF.