You may recall a few months ago or maybe even a few years ago, I had completed some Nickel Plate boxcars and when the decals were dry, there was a "silvery" look to them. Upon inquiry, I found that was indeed called "silvering" and it happens on some surfaces when the decal doesn't get completely set down to the finish on the car.
A little more poking around on the Internet convinced me that was indeed what was going on. Even though I used MicroSol before applying the decals and MicroSet and Solvaset after their application, the decals still had that look to them.. The solution seemed to be to apply decals to a glossy surface. Well, my friend Ray asked me to submit an article to the NKP Modelers' e-zine since he is the new editor. I sent him a photo of a car I did a while back and he thought it would be a good candidate for a rather easy project. The only problem was that it would be hard to do some step-by-step photos after the car was built. So I decided that my layout would have a pair of them and went to work.
I'm not going to describe how I built the car since I've covered that in a post a long time ago. But this time around, when it was time for the decal work, I first oversprayed the car with a couple of light coats of Gloss finish. After it dried, I applied the decals using my normal technique and the results are so much better. If you look closely at the Nickel Plate Road, you can still see a very small amount of silvering but nothing like the ones I did a while back. So I think the suggested solution works out well... make sure that you are applying decals on a glossy surface to avoid silvering.
The only downside to this is that the paint does build up over time so a couple of coats of primer, a coulpe of coats of the car's finish color, a couple of coats of gloss, the decals and finally a couple of coats of matte finish and some of the finer detail starts to go away. So my advice is to be sure that you are applying thin coats of any of the paints AND to do a little experimentation to find out if the particular paint brand you are using goes on thin or thick. "Usually" hobby spray paints user a finer pigment than the normal household varieties. They are more expensive but tend to go on a lot nicer. The choice is yours but it may well be worth the extra $$$ to get a nice finish on your models.