Acrylic clear should be fine over enamel or lacquer paint.
Did you do anything to the helmet before applying the clear? What enamel did you use? what brand of clear? What was the Temperature?
You cannot apply lacquer over enamel, or the same thing will happen.
I am sorry to hear about you unfortunate problem.
Been there.
Let us know if we can help further.
Thanks,
Robert E.