There are some 2-part epoxies that are low viscosity (check out West Systems 207 hardener).
I used some for top coatings as it is designed, but have used excess for general fill uses and was pleased to see that the bubbles cleared out before it set. Mixing without aerating much and a careful pour help.

It's egg-pensive but even with the small quart kits of the 207+resin you get a lot.