There are two reasons why your casting may have air bubbles in it.
The first reason is because your mixing process introduced air into the mix. To minimize this, try mixing by hand with a tongue depressor or Poly Paddle for larger mixes. Stir gently, moving in a figure 8, being careful not to whip air into the mix. Be sure to scrape the sides and bottom of the container while mixing. Pour gently as well to minimize turbulence that may be created by the pouring action.
The second reason you may have bubbles is because the design of your mold creates some undercut conditions that trap air during the pour. Try tilting the mold during and/or after the pour to let the trapped air escape. Some call this “burping” the mold.
If you are still having trouble with bubbles, casting the part under pressure will usually solve this. Unfortunately, if you didn’t make your mold under pressure you may have some bumps or pimples on your casting. However, these can usually be sanded off much easier than trying to fill a bunch of bubbles.