Here is a picture of my custom Legion of the Damned squad that I painted couple months back.
As you can see they got this awfull frosting all over. This is due to GW Matt Varnish has been sprayed on:
a) and the can hasn't been shaken enough
b) humidity in the room was too high
c) it was a bit too cold
But don't worry! There is a way to get your models to like at least like this:
How did I achieve that? Simply follow the steps below with utmost care! In the end they are your beloved models, aren't they?
Note: All varnishes I used were paint-on varnishes. We wouldn't want to repeat the mistake, now would we?
Step 1: Try warming up your models. Sometimes, on rare occasions, that might be help enough. For some reason the heat can melt the layer of varnish or something and restructure it... I don't know the technical reasoning for this, but it works. If it doesn't, don't put your hair dryer away just yet.
Step 2: Apply a gloss varnish (any make will do but I noticed the GW and Vallejo work best). Before you do it, warm a model just a little bit as in step 1.
Step 3: After the gloss varnish has dried out, heat it up again and apply matt varnish. This should fix your problem.
Caution: Do not use a hair dryer TOO CLOSE to the model, you don't want to melt the plastic. It is better to keep it in the heat for longer than melt it straight away!
OK, I hope that helps! :D