imho, you would have been better off to cover the plane with cub yellow Solartex and just painted the cowl and any other parts that were not covered.
there is no way I can tell you how much paint or primer you are going to need, to many variables. you will need to sand the yellow you applied to cockpit area.
Primer is not a sealer. Primer is an adhesion promoter for the paint. there are some high fill primers you can get in rattle cans, spray the high fill primer then sand most of it off, then repeat until the area is smooth. on unsealed wood it takes lots and lots of coats of high fill primer and sanding in between coats to get a smooth surface. it is always best to use a sealer before using a primer or color coat. without seeing your plane it is really hard to try and advise what you should do to finish the painting. I will say this, I will highly recommend that you start over on the elevator, as in remove the painted Solartex and repaint it with a light coat of primer followed by a couple light coats of cub yellow. Right now you have heavy coats of paint on a part of the airplane that should be kept as light as possible, and anything else you do to the elevator will only be adding more weight. Cubs are generally tail heavy airplanes and extra coats of heavy paint on the elevator will only make it more tail heavy. Recover the elevator, I know it will not be fun, but it is good practice and you are learning a lesson. I always recommend that people make a small practice wing panel, cover it and paint just the way you will be covering and painting the model. this will give you a chance to see how your finishing products will work together and give you a chance to make changes before you start painting the plane.