Here's some theory on adhesion, based on my training in aircraft metal bonding.
In this instance we can compare a glossy and a flat paint finish with an untreated and anodized/etched aluminium surface respectivly.
Depending on how fluid the adhesive is, in this case the liquidized salt, the salt will adhere to the surface in different ways. The basic rule is for more adhesion, the more surface area is needed. The reason why an aluminium surface is anodized or etched before bonding is to create a porous surface. If you were to take the entire surface, including the tunnel lining created by the anodizing/etching, then the surface area would be hugely increased. Now the more fluid, the better the adhesion is going to be because it can creep into the rough surface. When it dries it creates a very strong bond. Basic rule for salt is that its not made too wet, otherwise it'll never come off (completely) again. The reason why a gloss coat is laid down before decals is to do the same. Create a large as possible undersurface. On a flat finish the decal basically sits on top of the microscopic paint mountains. The silvering you see on a decal is just air underneath the decal. By using a gloss coat and overwetting the salt will also make it stick like a bugger. Imagine a pot on the kitchen counter that needs drying. If its sitting in a puddle of water and has a flat undersurface you get somethng called hydraulic lock. Its not too different from a suction cup. As there0s no air and no air can get there, the underside of the pot is vaccuum sucked to the counter with a perfect seal. It takes a fair bit of force to break this lock. Imagine that with something that dies up and solidifies.
In other words, you only ever want to use enough water to just make the salt stick. If you don't use too much water it really doesn't matter if you use flat or glossy paint. In fact, by exploiting the silvering decal phenomenon, you can get salt to hang on until you rub it off and get everything of ( I just rub it off with my finger..), so flat is beneficial, going over what I just wrote. Besides, it also speeds things up because you don't have to wait for an extra layer to dry..
Hope this helps..