Hi Willel,
Here's what I think you can do to smooth out your pours
1. If you're using a carved piece of wood as you master make sure is not to porous and that it gets a good smooth sanding. (I don't use wood so I can only speculate this might be a problem)
2. look at your mold, does it have these marking in it? If so try to eliminate anything on your master that might be causing this.
3. When you mix the resin and micro balloons mix one side of the resin with the balloons and let all air rise to the top before mixing it with the other side, this allows for a more even mixture.
a. if the side that has all of the voids is the top of the mold or the side that you pour the resin I would think that you are mixing to vigorously causing air to get entrapped which rises as it cures.
4. try to get a pour that requires almost no sanding. Like gon2long explained micro balloons are tiny glass spheres, if you sand them they rupture and leave very small voids. They will show up after you paint and clear.
6. Make sure your resin is clean of any residual residues from the silicone or resin, this will cause fish eye problems when you paint and clear.
5. If all else fails pressure cast them.
There are some steps that help you cover them up once they are present, use an acrylics base primer to help fill them, epoxy to self level them or auto fillers like bondo. (search the forum for how to's)
If pouring lure is something you want to do on a regular basis I would avoid trying to use to much surface filler and just make sure your process is dialed in, when I first started pouring resin I had similar problems, I constantly searched for a magic solution but In reality you have to make sure every step in your process is done properly. Once you dial it in it's not too difficult.
Good luck,
Pete