Sounds like incomplete wetting of the filler powders. The pour is not under ressure after it's cured. Even if the bit was hot, the epoxy pour is not frangible, so would not turn to powder. Think about drilling out the hole oversize, filling it with thickened epoxy, and redrilling. Don't mix it too thick, you want to wet out whatever the problem is in there, but with some epoxies a moderately large thermal mass can cook off without filler in it, cracking, smoking and othewise misbehaving. The filler works to moderate the temp spike.

> I drilled into the end pour tonight for a rudder installation. When I
> drilled the pilot hole and pulled the bit out, it poofed powder about a
> foot high! The bit was not hot. Why would it poof? Is the solid pour under
> pressure after it is cured? I think it was more poof than just hot air
> trapped, especially as the bit was not even warm. Perhaps there was a void
> that had compressed air somehow? The filler is a mix of fine sawdust and
> microballoons.