if it was me advising ...id say dont paint over it ,more product will increase the chances of this
if you must use celly , then bare metal it , repair whats needed then epoxy prime it , this will be a ideal base that once cured its done , no moving around in the sun and shrinking etc , sand that and celly it
You can't "bare metal" fibreglass.
In fact if not properly cured the shrinkage on the fibreglass or even expansion in the sun being a dark colour that gets hot, or even flexing could contribute
