Mark,
Are you sure that the Rustoleum clear coat is enamel based? Enamels, if I'm not mistaken are petroleum based, so you shouldn't have a problem with covering the Testors which is also petroleum based. Now if your clear coat is lacquer base then you will have a problem if not done carefully. Lacquers are "hot" and will eat into petroleum based paints.
I use pretty much the same proceedure as you described. However, I use Krylon Fusion spray paint. Then after cresting with the Testors I spray a VERY fine mist of Krylon clear lacquer over it all. It is important that the first coat of clear be sprayed on very lightly so as not to make the Testors cresting lines run. That first fine coat will set everything and then you can go back over it with a normal final coat of clear.
One other thing, it's important to keep your Testors cresting lines pretty thin. By that I mean keeping the paint thin. Thick paint will run much easier than thin.
This is what works for me. You might want to give it a try. Good luck!
Brett