Andromeda

you'll need to use a gloss clear coat that is of the same type as the paint used on the model.
A model being on display (public) at a fair I would advise against spray cans. if possible try find some one with a compressor and spray gun (or an airbrush) to apply the clear coat.
Spray cans can't be controlled, as the paint is used up in the can the pressure drops and gives an uneven finish.