Believe it or not if done right paint can come out just as light as ultracote/monokote. But it takes a lot of learning and or skill to do it right... To paint a surface you need to "glass" it 1st. That is you put a layer of EXTREMELY lightweight fiberglass over the entire part of the airplane that you intend to paint and apply it with a thinned epoxy or fiberglsas resin that goes on about like water. Once it dries you sand it out with a very fine sandpaper and prime, sand some more until you get it slick and then paint. If done correctly you will come out with an extremly lightweight, strong airplane.

But its not easy...

Here are some examples.