A quart of Minwax Polycryllic should be enough. If it is difficult for you to get, I'd order two quarts, just to be safe.

Apply the poly in thin coats. If you lay it down too thick, it takes forever to dry. This will gum up your sand paper as you prepare the surface for primer.

I suggest that you wait at least a week to prime the surfaces, after applying the polycryllic.. If you prime it sooner, you run the risk of trapping moisture under the primer. This causes blistering, when the model is completed, and sitting in the sun.