First off, Welcome Aboard! Saw your other post, Be sure to throw something up in the New Memebers Forum. We're a nosy bunch and like to know everything about whom we're sharing informations with.
Ok, Secondly, Your'e over building. Studio flats don't need to be out of 2x4. You can frame them with 1x4, or if you want to have a lot of fun, go buy 1x8 rip it into 2.5" strips and use that for the framing. If you can find a decent supply of 1x3 "firring strips" those work well to. As for cover 1/4" luaun is my panel of choice. You can get away with 1/8" but the tend to buckle a bit more when painted.
As for
dutchman-ing the seams. Your'e right traditional dutchman techniques don't work so well on studio flats. First, when you construct the flats, be sure to put a slight < very slight > bevel on the outside edge of the flat, make sure to knock of any "edge crumbs". When you assemble the flats on stage start from the bottom work your way up with two people screw the together every 24", the person on front is responsible for geting the most even seam possible. Once all is done, grab a tube of "good ol' Alex" , Latex Painters caulk. Don't use silicone, only latex. Apply the caulk just like you would if you using joint compound. Viola' Instant flexible seam. The bevel you put on earlier will allow for enough material to inflitrate the joint. When striking the set you can just run a matte knife down the seam to pull them apart. The nice this about this technique is that if you pre-assemble your set in the shop you can pretty much do all the final paint
etc. then carefully slice the flats apart, move them onto the stage and get a very nice seam by matching cut lines, without having to repaint. Alex can also be tinted with standard latex colorants, and it can be mixed with paints.
If all else fails close one eye walk away 15 feet and say, " Yeah, they'll never see it from the Audience."
If the director says, "Yes they will." then you say," If they're looking at the seams, then somebody else isn't doing their job."
No wait, on't say that it really pisses off the director.