What Future does, and other clear acrylic liquids, is to fill the tiny pits and scratches on the surface, and that's what gives the surface some clarity. The rough, pitted, scratched surface scatters light, while a smooth surface allows more light to pass through without refracting (oh, no! Science!)
For surfaces etched by glues, for example, you can polish the damaged area by sanding with successively finer grits, then use the clear coat.
And generally, many modelers will dip clear parts in a clear sealer, like Future or other non-modeling product, or in clear media from hobby companies. Same thing-this will tend to fill any fine pitting or scratches and make the surface clearer.
A note to check your sources, though, depending on the subject, the scale, etc. You can find plenty of photos of aircraft with windscreens and canopies that look rough or hazy, because the actual piece of plexiglass was worn and weathered. Check your sources and do what you feel is best for what you want to achieve.
Something else to keep in mind with clear styrene, generally, is that it can be more brittle than colored styrene (Chemistry!) So as others have mentioned, it can be better to use a fine razor saw to remove parts from the sprues, than to use a sprue nipper. Cut as far away as possible, sand and file carefully.