I just finished trying Rustoleum over Ultracoat and it seems to have worked fine, but there is a problem.  The Rustoleum wicks under masking tape.  With a dope finish the trick is to seal the tape line with clear dope.  Is there a similar trick that works with Rustoleum?  Note that I did use hi-tac 3M tape and ran my fingernail over the tape edge before spraying.

Allen Brickhaus has been using Rustoleum over film for years, and recommends that you go ahead and use masking tape, but peel it off when the paint has only dried for an hour or so.  Then take a Q-tip and some mineral spirits, and very carefully clean up the stuff that leaks under the tape.

It works a charm.  It even works over Rustoleum, if the coat underneath has had a week or two to dry, and if you don't get too enthusiastic about scrubbing.

Spray lighter coats, that way it will set up before oozing under the edge of the tape.

This may sound weird, but I've been using Rustoleum protective enamel in the quart cans and applying it with a foam brush.  This is over Zinsser primer.  I sand after every layer, then just let the last one dry.  It looks pretty good, and when it is time for trim details, I just mask off the Monokote, scrub it with a 3M pad, wipe it down with actone, then brush the Rustoleum on.  Works a treat as the Brits say!

If someone will tell me how to post pictures, I'll try to put one up.