Mix the balloons THOROUGHLY with the resin ONLY, then after they are thoroughly mixed add the hardener.

I frequently use micro balloons with Bondo for making fillets (cheaper than the SIG epoxy-lite ). I do my initial sanding about 10 minutes after it kicks/solidifies and the final sanding the next day. Sanding the next day takes a little more effort, but 180 - 220 OPEN COAT paper handles it just fine.