The factory paint is a base coat/clear coat system, which means the color is the base coat, and it needs a compatible clear coat sprayed over the top of it. Even the denim paint is a base coat/clear coat system.
The best way I can describe a single stage paint is a rattle can in gloss black, which will very rarely look like a base coat/ clear coat paint job. Prep is everything in a paint job. If a single stage paint was used on your parts, then it probably wasn't made to accept a clear coat over the top of it, and it might be having some type of chemical reaction. Either way, I wouldn't be happy with the results you ended up with.
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2015 Denim Black RG, FM ss ceramic coated 2-1-2 pipe, White Bros Original E Series Mufflers, FM ac, PV Tuner