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1) no. the 3m filters, while they work pretty good to start, clog quickly and then the powder starts to pull through. my first booth filtration system was an mdf box (4'Wx3'Hx2'D) with a 1200cfm squirrel cage blower mounted in it exhausting through the 2' side, and the 4' side had two of the 25"x20" high efficiency filters. it worked ok to begin, but like i said, as soon as the filters load up, the powder's going through - and through the blower (still never killed it or blew up). there also was not a lot of air moving through the filters.

LESSONS LEARNED: A) furnace filters are not good enough to exhaust inside. B) squirrel cage blowers lose efficiency VERY fast when the flow is restricted. radial blade blowers - also known as high pressure blowers - are the ticket.

2) it's probably fine for both? we just started using cyclone separators (one for the shop vac and one for a 55gal drum) and they're amazing. no filters to clog or replace, no loss of flow, no moving parts (besides the blower that you'd have anyway). do a search for "cyclone separator" on here and you'll find a link i posted to the ones we bought on ebay. they're VERY reasonably priced.

3&4) yeah, galvanized steel will work fine. my first booth was a frame made out of pvc (sturdied up by criss-crossing steel cables), the floor was mdf, and the walls and ceiling were plastic 6mil disposible painters tarp duct taped to the frame. "HOW can you ground THAT?" turns out it doesn't really matter. whatever your parts hang from need to be grounded. if you look at one of the lower end nordson powder booths (the window style, not walk-in), the three walls and ceiling shell is polyethylene or something. the metal bar where you hang your parts is grounded. but yeah, use screws or aligator clips or whatever you want. just take a multimeter to everything to verify that it's a complete circuit to the earth ground (earth ground can be a piece of conduit since outlets are grounded to the j-box which is connected to the conduit. again, just use a multimeter to verify continuity.

if any of that is unclear, lemme know!