I also apply directly over the metal finish. In fact, clear coating is only required over flat paint. If you have gloss paint you can decal directly over the paint.
The question comes up then about sealing the decals after they are applied. I seal decals applied over paint with a gloss or flat coat. However, such coatings can change the appearance of the NMF. So I do not clear coat metal finishes. So far I have found that adhesion to decals over such finishes is good enough I have had no lifting.