Ok in vocationalschool were I took autobody in high school, My teacher gave us a test on spraying laqure over enamels. Yes it can be done!!! you have to spray couple of light coats,that acts as a berrier coat. then you can start spraying it,but not to wet!! Got a A on that test. I'd shot over acrylic enamels with urothanes paints before. If the enamel has hardner in it and its old ,then you can paint over it.Just use paint sealer over it first!