Ron, I've had the same thing happen to me when using Dullcoat. Many MRR's have--there's been quite a bit written about it. I think you may be right about the problem being the result of long shelf life. Trouble is, even when buying it "new", you have no way of knowing how long ago the clear paint was manufactured. I've also read that the humidity when you were spraying might be a factor, as well as the type of paint already on the car--another thing you can't know.
The "dissolving" of the chalks is common when using any clear fixative, BTW. You just have to apply more chalk and repeat.
Unfortunately, I don't know of any way to get rid of the "frosting". If you'd painted boxcars or covered hoppers, they might still look good frosted (i.e., "dusty"). But sure not open, coal hoppers. Try applying more dark chalk over the "frost", then use another clear fixative. It'll be heavy weathering, but maybe it'll look okay.
If there's a lesson to be learned here, it's to ALWAYS test "new" paint before using it. /Tim.
