Try using clear resin. This stuff isn't like paint since it has a higher solids base than paints do. If you want a truly wet look you'll have to pour this onto your model, but not onto the canopy. Brush it over as soon as you get your resin mixed. Mask where you don't want resin to get into like the cockpit edges where the canopy rests. It may sound stupid at first, but I did this accidentally to a piece of wood. It still looked wet even after a half hour which it actually cures in half that time. Paint dulls slightly once is has gassed out. What happens is that the solids in it settle into the rest of the paint formulation, and therefore once dry it doesn't truly have that wet look because of the chemicals that are used to make the paint "flow out", or spray on smooth, and the driers in it escape through the paint layer this makes it slightly duller when it cures. This is why paint polishes were formulated.
Resin doesn't have all these chemicals in it as it's catalyzed with only a hardener. It dries fairly even when brushed, and leaves no real brush strokes. So if you want a truly "wet look" this will do it. You'll want to paint over your ground surface with the same resin if you're going for the "It just rained", or the "it's raining look". Or what you can do if you're trying to get the "wet, but not raining look" you can try to paint it with a satin finish, then polishing it with Turle Wax brand polish to make it wet without being soaking wet. The way it works is that these planes are painted with a flat finish, so if you're going for a wet, but not soaking wet look it'll be a little softer in the wet appearance than it will if it's raining. Without using some type of waterfall device in a closed box/cage this is the only thing that I can think of that will work reasonably.
~ Cobra Chris