Hi Blink, Sheeted areas do not need
covering in mylar but you will find a difference in shade of the tissue where the mylar ends, so to speak. If the sheeted areas are small (such as infill at the nose or tail) then I cover them with the mylar, but on areas such as wing leading edge sheeting just the open area. Of course, the Ajax will not have large sheeted areas so you could cover the whole thing with mylar. As to treating the woodwork first, it seems best to seal it and I use diluted varnish for this but another way (apart from dope) is very thin Balsaloc. Either way works well but if you use varnish it's best to sand the structure when it's dry. Thus you start with a very smooth framework. Remember when applying mylar to allow an escape route for trapped air, ala solarfilming. A few pinholes is all that's needed as air is pretty thin!
Followed your build with much interest and amusement at your sense of humour.
All the best with the covering. If this your first attempt then I would very respectfully suggest you knock up a trial frame of some sort and use it to practise. Also, on the model, a good place to start is the fuselage under side. BOL.