Like I said earlier I can't believe the EPA would give a crap less about what some guy was doing with his touch up gun in Heyburn Idaho. They have much bigger things to worry about. The local agencies, most likely fire dept is the one that will come in and kick your butt.
I don't remember the particulars and to tell you the truth they would likely be changed now anyway. But I had no booth at my little shop and use to keep things pretty small, the rule was something like spraying under 6 square feet was legal something like that. I had an insurance adjuster sic her dad on me who just happened to be the fire chief in town.
He rolled in and told me I couldn't spray without a booth, I brought up the 6 square foot rule and he brought out his book and let me read a page, it talked about needing the air exchanged every certain period of time, I forget what it was. And I remember looking at him and saying "Come on, that's a booth, it says you don't need one but that is what it's describing" and he just smiled while keeping eye contact with me closed the book with his right hand. :evil:
He knew his daughter was a beach and he didn't give me too hard of a time. But he COULD have. The fire issue is much more an issue than the vapors that escape. We are talking about an explosive box here, that pretty much trumps some thinner evaporating into the air.
And these laws are so confusing even the people who carry around the little rule book don't have a clue. I had a fire inspector tell me I had to have an explosion proof cabinet that held a certain amount of paint (I think it was 10 gallons, but don't remember) I went out and bought this five hundred dollar cabinet only to have the next inspector a year later tell me I didn't need it and the other guy was wrong!
That was 30 years ago and I just recently brought that cabinet home from where I now work. I had sold it to this shop across town when I closed mine never imagining I would be working for him, it's not like I knew him or something. So here I am and the other day they were tossing it out so I grabbed it for my garage. :thumbup:
Anyway, honestly, not many of us can help you with this project because the laws and ordinances in our towns can be completely different than yours.
A booth is simple math, you need a fan that will exchange the air inside of a box in a certain time frame. It is a simple formula of how big that box is and how much cubic feet of air the fan moves. The air intake to that box has to be big enough that the amount of air needed can enter the box.
I worked at a truck painting place with a fan that was about three feet in diameter and sounded like a friggin 400 horse small block when it got cranking! The booth was about 25x25x50 and that fan was moving some air!
A basic old cross draft booth could be pressurized I imagine but generally it is simply sucking the air from the box and you put a hole at the other end of the box with filters for an air intake so you don't suck dirt and small birds into your booth. The air exhaust also has to be filtered so the paint particulates can't leave.
But honestly, you had better do your home work as building this thing and then being told you can't use it would suck.
Brian