According to Smooth On, the 300Q doesn't need any degassing, same as the 320 that I use. However---you can never go wrong with degassing if you need to----in which case, you would need a resin with a longer pot life. I have never used a vacuum chamber, and have had pretty good luck with my castings. Every now and again I might get a bubble, in which case I mix a little dab of Bondo to fill it.
Try the 320 next time. Good luck.