It is a very small picture but what I can see the paint has wrinkled. I'm surprised that this happened since all the paint products you used were Rustoleum. And you waited fed days for paint to dry/cure. Usually the same brand paints and clears are compatible. I don't think scuffing before clear is needed, but it probably did not cause the problem either. It was a good move to test the clear on a test piece.
You might be able to find a more compatible. I'm in N scale and I use hobby paints, so I can't really offer any recommendations for a safe clear coat. I usually use Testors Dullcote and Glosscote. Just maybe try some other brands. If there is a water-based clear available, that would most likely be safe.
