We typically recommend a 5 or 6 cfm pump. The cheapest one we have found is a Robinair from www.tooltopia.com. Here is a direct link to the site (you might have to cut and paste it in): http://www.tooltopia.com/index.asp?PageAction=VIEWPROD&ProdID=9012
The part # is ROB15600. I believe that the $252 is a delivered price. We recommend them for our Water Clear Urethane and Flex 80 urethane rubber but mostly for our silicone rubber mold making materials.
We sell a pretty decent size Vacuum Chamber that is approx 10" in diameter and 12" tall (probably overkill for the amount of materials you typically mix up). The Robinair is able to pull 29" of Hg in about 1 minute. We've found that 29" of Hg is the magic number to make rubber and resin degas very quickly. 28" of Hg works but may take too long. For whatever reason, when you hit 29" of Hg the material starts dancin'. The lower the cfm, typically the longer it takes to reach the sufficient vacuum needed to effectively remove the air. Also the smaller the vacuum chamber, the less volume is in the chamber, and the less time it will take a pump to pull down the same amount of vacuum.
I am not a pen turner but am very intrigued with the process ever since a couple on this board starting teaching me. They started by asking me a few simple questions about our Water Clear and now I think I've asked them 5 times the questions they've asked me. But from my perspective, I believe that yes you can simply mix the resin, degas it and slowly pour a perfectly clear blank that is ready to turn without any pressure. I say that because this is the exact process most people use when casting Water Clear parts with our resin. Since most mold makers already have vacuum and may not have pressure, they simply remove the air from the system before pouring. The only thing you have to be careful about that the pressure takes care of for you is the air bubbles formed when you pour. That is why as Fred_erick mentioned, tip your mold on an angle and pour it slowly down the side to avoid the waterfall affect of introducing air while pouring just as you would a tall cold one.
Mike
Alumilite
800 447-9344