Possible reasons:
Mix is not quite right (i.e., too much hardner or resin)
Not mixed well enough
Too much heat, caused the epoxy to dry too quickly
Did not allow enough time (usually 24 hours) for the paint to cure
Something on the plug, like hand grease
Did not apply an even coat of epoxy to the plug
A combination of the above. At least those are the reasons I sometimes get dimples in the clear coat
Jigman
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