Look up some explanations from Bill Byles, and noted full scale aircraft finisher, who is a member of Stunthanger. Nitrate is used on full size aircraft finish applications. Think of it as a sort of primer. It comes in tautening and non-tauteing types. Free flighters use it for rubber powered models and gliders because it is supposed to be a bit lighter, and not fuel resistant, which you do not need on rubber models and gliders. It adheres better when applying covering with dope, and fills faster during build ups. On full scale coverings, they use a tinted non-tautening nitrate to apply the pinked strips over rib stitching and such to make sure it's held down securely, and it's tinted so you can see where you have been with previous coats.. Then at a certain point the butyrate comes in for final build ups and colors. Silver is applied under the colors as a anti UV measure, I do believe, and like on a model can show you where any blemishes or trouble spots may be so they can be addresses. The silver makes a good under coat for almost any color also.
The whole process can be done from start to finish in butyrate alone as Ty points out. The same can be said of Nitrate if you are building an electric model and are weight conscious and don't need a fuel resistant finish, just don't pit you model aroud anyone with an IC powered airplane that likes to run his engine in the pits!!
Type at you later,
Dan McEntee