Kenny,
Base clear is probably the lightest paint method you can use. If you are doing a flat finish, as Tam said it's easy to keep it light. 2 coats of clear is all you need, you can get away with one coat if you're good at spraying.
When I did Dean's Kingcat I added 24oz in paint, when you consider the size (probably close to the same amount of surface area as your 18) and the fact that it was a high gloss finish sanded out and buffed (which required 4 coats of clear) this is pretty reasonable. The sanding of the clear probably reduced the weight a bit. You can't really judge what the finished product will weigh based on what your raw paint weighs either even based on the amount of paint lost in spraying (70% as sean said) as most paints will evaporate a solvent during cure. Especially base. The RAW weight of paint on Dean's Kingcat was probably about 4 pounds, to end up at only 24 ounces in the end shows how much of that weight evaporates during cure.
Dean's jet also used a lot of pearl and Kandy colors which increases weight due to heavier base colors. White and Red are the 2 heaviest colors, the pigment is heavy. When you add metallic or pearl flake it adds a bit more too, it may be negligable, but we're talking weight here. Dean's jet was first entirely sprayed with silver (a heavy color to start) then gold (another heavy color) then Kandy red, Kandy pearl purple, and Kandy blue. Then the airbrushing took place. For the checkers this all had to be based in white again. As you can see, I used many colors, and ended up with very little weight for such a large aircraft

Here is the kingcat in question





I'm just putting this picture in to show the depth of the clear, this isn't done with one single coat