With a siphon feed blaster, such as the one at Sears, it will take you FOREVER to blast your car. These type blasters are for very small projects, they just don't have very good coverage. I would recommend buying one of the pressurized units from a place like Harbor Freight or someplace comparable. These units initially cost a little more, but the time savings will be well worth it. Another consideration is your compressor size. The pressurized sandblasters will need a larger compressor than the siphon feed...and I am sure you probably already know about being careful with your body panels when blasting, because if not done right, you can warp them.
When I think of a sphon feed blaster, I think of a blasting cabinet. The pressurized models are more for what you are talking about doing.