That condition is called "fish eye" . when the epoxy pulls away from various spots becasue of surface contamination (usually oils etc).
Give the plug a coat of clear spray before epoxy next time or be sure you do not handle it. Keep the surface clean!
The plug that is bad; Give it a light sanding and wipe it down with a rag dampened with alcohol. Let the alky drie and give it a sencond coat of Devcon. The epoxy should level out in theose voids.
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