Normally any warm air you have in the house will rise up from the basement and work it's way outside through any leaks you have. The cold air will come in from the basement to replace the air that went out. You will be trying to force the warm air outside from the basement which will be pulling it down in the opposite direction that it wants to go. I don't know how it'll work for you, but I'd swear that my heater ran less when I had the fan going. I just opened up the plastic a little bit to get only a little flow going outside with negative pressure on the room. The plastic was sucked in quite tight. I had it taped and stapled so it wouldn't pull free. The real small desk fan (the kind with a clamp on it) was plenty to do the job. It was nice that I had an outside door to the area so when I wanted to seal it up and start doing epoxy work I would have to tear the plastic open every time I wanted to go in and out of the room.

Another thing to consider is that you don't have to run the fan if you have the plastic up on the few days you're sanding, and there aren't all that many days that you'll be epoxying. Most of the time you'll be gluing wood strips. By the time you're ready to epoxy it'll be spring anyway.