I've done this with good success: Seal up with plastic as well as you can, then place box fan (or two) at rear of garage for exhaust. filters in opening at the front (no fan) will allow clean air to be drawn in. you don't want a whirlwind in there, only some good cross flow so that the stuff in the air will be evacuated. Wash your booth out the night before you shoot. DO NOT do this immediately before as water droplets will invariably get splashed up on the body. DO wet it down outside everywhere. You can also zip tie filters to the exhaust fan and put a fine spray hose-end sprinkler near the exhaust exit just outside to help wash the paint particles out of the air as they go past. This is a must if you have close neighbors or don't want any overspray on anything.
Your situation does not sound so great for cross flow. You may have a cloud in the back of the garage while the front is nice and clear. Good luck.
