Here is a thread where we talked about it quite a bit.
http://www.battlegroundgames.com/forum/index.php?topic=5271.0Personally, I use Testors Gloss Coat on every model, then after 24 hours Testors Dull Coat. If you read through that thread, you will see the reasoning, the process and the best place to get them.
As for your dusty models, you might be able to save them. It happened to me before on my Stompa before I switched to the method I am using now.
Gloss coat the dusty model. Wait at least 24 hours for the gloss coat to cure. This will basically undo the matte finish, then hit it again with Testors Dull coat. If you want the science on how/why that works you can look it up on the internet. It is an interesting look into the optics of surface coatings.