I've seen some people doing the zenithal spraying, looks like a useful technique.
I use it all of the time. I undercoat in black with a halfords spray, then do another colour sprayed from the top down and not too heavy. Leaves all of the shading in and gives a good guide on where to paint
I would ask the person who painted it for you what they used, and then use another coat of that on it. Safest way really.
I would advise against a spray varnish, as you can easily mess up a nice paintjob with it.
I can recommend Vallejo Varnish. Matt, Satin, or Gloss, depending on what you need.
@JT People very much still use white. Or Black, or Grey, or a mix of all those sprayed in a zenithal lighting fashion.