Ideally, the fan will seal to the wall where it passes through.
This is the fan I use, with some louvers on the outside to minimize air leakage when the fan is off. Mine is about 30 years old, but it still works great. I have used it in 4 different spray booths in the past 25 years.
http://www.grainger.com/product/DAYTON-Exhaust-Fan-10E016?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/10E016_AS01?$smthumb$
For air filtration to the actual spray booth, to seriously minimize risk of contamination in your finishes, This filter:
http://www.ebay.com/itm/ATMOS-TECH-...417?pt=LH_DefaultDomain_0&hash=item2ebc599659
With this filter in front of it:
http://www.ebay.com/itm/3M-E736-V-B...442?pt=LH_DefaultDomain_0&hash=item4adb256b0a
and a fabric pre-filter of 200 micron efficiency in front of that,
with one of these fans in between the 85% ASHRAE filter, and the HEPA filter:
http://www.canfilters.com/fan_metal_home.html like an 8" H.O. or a 10" H.O., or if you want to move some serious air, a 12" Max-fan (rated at 1700 cfm free air, 1300 cfm at 1.5" wg, and still moving air at over 3" wg! at max amperage draw of just over 4 amps!)
Some creativity is necessary in building a case to house the filters and fan, but this combo will deliver very clean air if your construction methods are tight, HEPA filtered air in quantity. You can deliver it downdraft style directly above your spray area, kind of diffused, to almost eliminate any dust entering the finish.
I have 4 of these 12" Max-Fans in my shop for various air filtration, air moving tasks, and one in a downdraft table that I built that cleans the air with dual industrial V-Bank 85% ashrae filters like this:
There is a prefilter in the downdraft table, above the V-Bank filters and under the table surface.
or just another air cleaner in my shop, for my lungs.. ( still being built in these pics, I then lined the entire box with some serious sound damping foam, notice also that the fan is decoupled from the plywood construction of the box, that black line around the fan box inside, is damping foam with a bitumen layer, friction fit with no wood to wood, or wood to metal contact. This thing is almost silent, and it delivers a LOT of HEPA filtered air into my shop)
One of the air filtration units in my shop has been running 24/7 for over 5 years now, just cleaning the filter element every month or so. In the very few times that it has accidently been shut off, I can see the difference in settled dust overnight.
When I walk into the shop next door, where they have no dust filtration, dust a standard dust collection system, I am SO thankful that I took the time to help deal with wood dust in my work environment!!! I highly recommend it! At least a good downdraft table!
For me, with the amount of lacquer that I spray now days, or say, up to 4-6 instruments a month, my priorities in engineering a spray area would be, in this order, First, that I can spray the finish without contaminants, Second, that however the operations and expected use goes, that I do not have to breathe any lacquer fumes in surrounding areas of my work space during the layup of coats, or during the curing, and lastly, that the lacquer mist is evacuated from the spray space in a safe manner.
Another consideration is what do you do while the pieces are curing? Do you want your C-130 style spray fan running 24-7 until it does not stink anymore? If there is no moving air, a freshly lacquered little uke will stink up a pretty good sized space for a couple days, at least. One of those 4" or 6" can fans and a little ducting, hooked up to a fan rheostat to turn the noise and the air flow down, running constant and exhausting outdoors, will do the trick. After the initial cloud of lacquer is gone, 'explosion proof' does not really matter anymore so these energy efficient, and potent fans are awesome..
It is an uncomfortable place to be, being a small shop but having full on professional finishing needs.... when Tru-oil just won't do...
Sorry for getting so far off topic, rambling on and such..