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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.
Hey, is Citadel's Purity Seal good stuff? I've read not so nice things about it.
I've paid good money to get some of my minis painted by people far more talented than I, and would not want them to lose their looks. Quite sure they already have a clear coat, but would like to do an extra round just to be safe.