Well, it should be filtered both ways. 'In' to the chamber for the ultimate finish without any dust in it, and the 'out' filters are paint pads to keep the paint out of the fan blades, but - Go look at the paint booth at an auto body shop, study the details, and you start to understand the scope of the problem...
I would just rig a simple "open" booth to catch the overspray, and don't worry too much about dust. Because a closed booth gets complex and expensive real fast. If you are going to have a person step inside an isolated room and spray around volatiile materials (ranging from flammable to explosive when atomized during application) you HAVE to follow the same safety design constraints.
It has to be non-combustible construction in case of a flash fire, and not easily collapsed on top of the painter and work by the pressure wave, so a lot of temporary methods like draped visqueen plastic over furring strips is OUT. You need a way out, you need explsion-proof lighting (or the fixture outside the room shining in through a sealed window), you need to design against static buildup that could ignite the fumes...
You could build a little paint booth at home, but the project would eclipse the model you are painting. It would work with a freestanding sturdy frame of 2X4 studs with light sheet-metal screwed on the inside, and a Lexan window on top for the outside light fiixture. Prehung house door or two for egress hung opening out, and ball-spring latches only so they can pop open to release the blast overpressure.
You need pro-grade fire extinguishers ready at hand, strategically placed, and enough of them to handle the volume of materials. And a garden hose in case that still isn't enough.
Your exhaust fan motors have to be outside the airstream, and make sure the static can drain from the blower wheel/blade. And ground the sheet-metal walls, and the hook or table the work sits on, and make sure static can drain away from the paint gun...
If you spray two-part catalyzed paint, you have to use real non- combustible paint pads on the exhaust system. (Not just furnace filters.) These paints get hot as they cure, and you don't want your paint booth to spontaneously combust on you.
And no spraying any exotic aircraft paints with nasty solvents that'll kill you (like DuPont Imron) inside any booth without a full postive pressure respirator rig. Bought, not cobbled together - there are places to scrimp, this isn't one of them.
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