I bought all the PVC and plastic to build a make-shift paint booth for my single home build 32 Roadster project. Then I came to my senses, waited for some nice weather, and shot the underside and firewall over a three day period outside in the driveway in Austin Texas in August at 7:00 am (83F temperature) on each day. I wound up with an acceptable paint job with no bugs! I will definitely paint the top side outside as well.

From what I observed in terms of mist (HVLP gun used, AU paint) my make-shift paint booth would likely have left me in a toxic dense fog so to speak.

Bottom line, I think you need serious good ventilation (filters and brushless exhaust fans) and of course a fresh air breathing system to accomodate painting with isocyanates.

Regards, Kitz