All the different types of plugging I've ever used had these "air bubbles" in them, and I just figured it was a by product of the mixing procedure.

 The older, overnight curing plugging seemed to cure slowly enough to allow the trapped bubbles to rise to the surface and escape, so I also figured the little air bubbles were caused by the faster curing time not allowing the bubbles time to rise to the top.

 These are just my theories. Anything is possible though. They coulb also be by-products of the chemical reaction between the syrup and the hardener. I always heard the shorter the curing time, the more violent the reaction.

 P.S. A friend of mine has some Donkey Kwik plug ( cures in 10-15 minutes ) and that stuff gets so hot that it has, we think, caused a few balls to crack.
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Slower mixing, slower pouring, then gently tap the sides of the ball once you get it poured in there to float some of the bubbles to the top, or to at least settle the material a little better.  You won't avoid all the bubbles though, and the bubbles shouldn't affect the strength or integrity of the plug.
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