Jorma:
Okkay, so I got my little piggy and the rider painted, and went ahead and varnished the guy with Citadel Matt Varnish, I have used the product before without any problems, but this time the varnish left a “frosted” finish, destroying all detail on the miniature. On the image below you can see the effect on my Tzeentch BSB but on a much lighter scale.
Lol halp?
torn:
Did you spray? If so its will be moisture from the air trapped between the model and varnish. Pretty common if you use purity seal outdoors and its not very dry outside.
ThorAxe:
Temperature also has something to do with it. Used matt varnish when it was way too cold, the model turned from detailed to looking like it was frozen solid. Kinda looks cool in its own way.
JMR:
Your miniature can be saved. Applying a bit of white spirit over the varnish should clear the frosting. Olive oil might work too. I believe applying gloss varnish clears the frost too. You can then apply some matt or satin varnish later to get rid of the shine once the gloss coat is fully dry.
Your best bet is to google around a bit though, because it’s a very common problem and I am not 100% sure if my advice is the best advice given. I know the answer is out there on the web though, 'cause I’ve stumbled upon it a couple of times already. Luckily I haven’t really had to use it myself so far though 
Ben Saunders:
It’s never happened to me, but I seem to remember reading that you could fix it by re-spraying the model again.
Spray Varnish frosting [Archive] - Privateer Press Forums
Pit of the Oni: When Good Models Go Bad
I can’t vouch for these remedies personally, since as I said it’s not a problem I’ve had.
Admiral:
It happened to a few models when I spray varnished them years ago with 'Ardcoat. Since then I only paint on the varnish.
snowblizz:
I've done the olive oil thing and it seemed to work. With so many potential variables YMMW. Haven't tried respraying after that though.
Your miniature can be saved. Applying a bit of white spirit over the varnish should clear the frosting. Olive oil might work too.
JMR
Bloodbeard:
I don’t know how to save that, but learned from this thread. After once killing an entire mordheim warband, I havn’t spray varnished since.
I use vallejos matt varnish and brush it on the models in a couple of very thin layers. Gives much better control. And drybrushing varnish is often enough, since all the recess and grooves in the miniature won’t get beaten up.
So… Back to buisness.
I entered the thread again to complement that minaiture - what a nice BSB!