Three thing come to mind.
First, be sure your paint is thoroughly dried. Too thick a coat can retain trapped moisture, so be sure to do multiple thin coats, and heat set each coat really well.
Second, try a coat of Createx Clear Gloss over the paint before you seal with the Garco. The clear gloss has a much higher film strength than their paints, and should protect the paint. Be sure you clean your airbrush really well after spraying the clear gloss because it will set up like concrete in your brush and be a nightmare to clean if you don't.
Last, do a dip in another top coat first, like a water borne urethane, to protect the paint. It will add a stronger layer, too. All urethanes designed as top coats are much stronger than the paints they protect.
I've never used MCU, but, from what I've read here, it has a small amount of solvent in it that can affect paint sometimes.
Good luck.