The main problem with clear coats is that they will usually re-wet the paint you apply them over, if they are of the same type. This isn't much of a problem with regular colors, as this does little to the color itself. Metallics, however, are a bit different than regular paints, and re-wetting the surface can deleaf the finish, causing it to cloud up.
Your best bet for clear coating is to let the paint dry THOROUGHLY. This can be two days or a week. Plain label Rustoleum takes a long time to dry, and often metallics do as well. The Painter's Touch Rustoleum paints are much faster drying. Plus, sometimes, you just get a bad can of paint. (ALWAYS test your paint on a scrap of material first, preferably the same material you will be painting.) Anyhow, once the paint is dry, laquer can be used as a clear coat. If the paint is thoroughly dry, the laquer will probably not harm the paint you apply it over, and can even enhance the shine of some metallics. Testor's is the best for this, as it was formulated to be applied over their enamels, and will not re-wet most enamels. Rustoleum Laquer works well too, but I have had it react with some cheaper brands of paint. Again, paint a scrap of material, let it dry a day or two, and then spray the laquer and see what happens.
Another option is wax. I sometimes use furniture wax to apply a good shine over surfaces. It's also has the double value of being a releasing substance, which makes it easier to remove stuff thet gets spilled onto your piece. It's not as scratch resistant as a clear coat, but then, most things that scratch the paint would scratch the clear coat too, so it makes little difference. Just apply the wax, let it dry, and buff it. Be warned, though, some paints will be dissolved by the wax, so again, test it before ruining your handiwork.