It depends.

If the airplane has any open structure, it would of course have to be covered with SOMETHING.

In the case of open structure, that would usually be a shrinkable film or fabric. Some need paint and some don't.

If the airframe is completely sheeted, you could just seal, prime, and paint, but giving it a layer of fiberglass will give it that nice smooth, well blended look. It will also remove any sign of woof grain and make the plane look more plastic or metallic.

And it's not whether the plane is large or small, it's more a matter of personal preference.