They have a solvent thats called glaze remover, next best thing to sanding. It takes the shine off of a base coat before applying another coat. Try a small batch (a corner or something) before you do a larga area. It may want to soften the base paint and may want to lift it or it may crinckle.
Adding layers to paint can be an iffy thing. They dry and shrink at different rates and the top coat can soften the base coat and cause all sorts of grief.
I've had pretty good luck with Vecto Urethane satin on painted rims (kind of a half gloss clear). Though I've never tried it on paint newer than a couple of months old. I haven't tried the Urethane gloss clear as a top coat.
Most of the Rustolium paints I've used cure slow, much like an old fashioned enamel, the older it gets the harder it gets (it keeps on shrinking and curing for years).
You might want to try a polishing compound and see if the dullness isn't just in the very top of the paint. Not rubbing compound, but polishing compound. Read the label, if it is Polishing compound for clear coat, it is likely fine enough for anything. I've got various kinds of Dupont polishing compound in different grits, you can often get some good results on fadded paint. I'd use a soft rag folded in layers (moist to almost wet) and keep in mind the edges of the hammer indentations is likely the first place to rub through, don't overdo it, good enough is often better than perfect. Trying for perfect often leads to catastrophy.
I've had pretty good luck with silicon based paint coatings (like wax) to freshen up an old paint job, many also have UV protection in the formula. I'm partial to black cars, they get milky easy, the silicon coatings often work well for me.
