My thought is that the Plasticote laquer was a semi gloss or even perhaps a satin. His intentions were good, but probably not the right product. Or, if it was a full gloss product, it may not have the UV protection of true clear coat and has dulled. You could try waxing it, but for longevity purposes I would probably clear it.
First thing I would do is take a scotch brite to an inconspicuous area and scuff it a little so it is real dull. Then I would shoot a couple of wet coats of CC on it and let it dry. This should give you a good indication of what it will look like going that route. If you are satisfied with the results, once the "test area" has sufficiently cured, I would scuff the whole thing and clear it.
If you are not satisfied with the end result, I would scuff the whole thing, making sure to sand out any imperfections with paper. If it is rusted, you may need to sandblast, but chances are if it is not you can just scuff the whole thing nice and dull. Cover any exposed metal areas with gray or preferably black primer, shoot two coats of black base coat and then clear it. Good luck