I haven't had to paint the underside of a car yet but I would jack it up if I had to.

What do you use for the walls? I'm trying to find a cheap effective way of spraying in my garage with adequate circulation and little dirt. I have a 36'' fan and I've thought about buying some PVC pipe, making a frame and covering the walls with tarp or something similar. I could probably get all the supplies for around $100. Any ideas on a homemade paint booth? What have you done in the past?