That depends more on the kind of model. Military models, both armor and aircraft, tend to be painted using flat paint. Decals have a tendency to "silver" as air can get underneath and cause problems. Typically you would apply a clear gloss coat, some use Future floor finish as an inexpensive alternative. Then application of the decals, followed by another gloss coat to seal in the model, followed by a flat coat (again some of us use Future with a flattening base), then finally weathering, if desired. Automobiles tend to be painted with gloss paints, you could get away with decaling before a gloss coat.