I've just sealed the whole thing using Vallejo satin varnish. It's turned out great, the colour hasn't dulled and the paint is now more secure.
I recommend Vallejo, it's the best I've used for a long time, it puts out a nice fine mist which settles onto the surface without leaving an 'orange peel' effect, and it doesn't yellow... but best of all, it's non toxic
DAZ