to answer your question about paint- go with an outside OIL based paint, something light colored to reflect the light (white)
if you want to use primer you can, or just have several coats go on. it is easier to paint the wood prior to putting the canopy together. make sure that the paint has dried for at least 24 hours before you put the canopy over your tank.
One other word of caution, be sure not to use any of the exterior paints that contain mold, algae etc inhibitors, if your paint should peel or flake for any reason, the tank inhabitants *really* don't like that stuff!
i just built a new hood for my lights thinking of using a white polyurathane , i think it is available. i'm at the same stage, hod is built ready for paint. thinking also of installing small fan to pull heat out, would like to somehow hook up to thermostat. anyone know about the white poly?