I use 2k automotive clear coats nowadays. They seem to cover Rustoleum, Duplicolor and other enamels without issues. It seems to have allowed me to sidestep this issue. You don't need a great gun to do this, I have an $150 gun and a Harbor Freight gun. The Harbor Freight gun is good. You also need a compressor, the hose, fittings, regulator and filter. And don't forget the respirator. That is a lot of stuff, if you don't have some of it already it could be more than you want to get into. If you have a small compressor, adding a 5 or 10 gallon air storage tank, can allow you spray for longer while maintaining enough air.

The other way to go is to get Automotive 2k clear in a spray can. It cost a lot more, but it provides a more durable finish.

Eastwood 2k Clear Coat

SprayMax 2x Clear Coat at Summit Racing

You still need a respirator, but you probably already have one.