I think this may be a good part of the answer. I was planning to set my homemade booth up as positive pressure. Can't suck in dust at any joints. Fan and motor in fresh air so no worries about spark proof or explosion proof construction. Perfect solution. Then I wondered if this is the best way to do it, why aren't commercially built crossflow booths set up with positive pressure.
After some digging, I found a couple booth engineering papers online, along with the answer to "why not". Seems smoke testing as is done in wind tunnel testing has shown a blow thru booth is likely to have areas with near zero air velocity even when the total air thru the booth would show a 100 ft/min average velocity. These swirling motions occur mainly across the top surfaces of the vehicle where changes of shape are most abrupt. OTOH, the same booth running as a draw thru and handling the same total air quantity doesn't have perfect air flow across the surfaces, but it's so much better that it doesn't present a problem with redepositing overspray on the surfaces.
Some theoretical stuff was presented to explain the difference in flow characteristics but it was beyond the scope of my 50 year old engineering degree. The smoke pictures were good enough for me.