At that size, using two sheets of WA-DW will still be somewhat expensive, and making the effort to hide the seam will be at least as much of an issue as working to optimize the drywall finish...if indeed not more, and with less effective results.
If you want gain to any degree, and there is ANY visible line from the edge of tape, it will show. All due respect to energyfun
, that is not a viable method of joining two sheets, especially WA-DW, if one is considering a 140" screen that would viewed from 13'
If you choose anything, choose to go ahead and finish out the Drywall. It's a common sense approach, and if it already looks reasonably good, all it should take is a long day's worth of effort. They say, "Time is Money", but if your capable of finishing out easily correctable flaws and doing joints correctly, I assure you that matching up a $200.00 PJ with $160.00 worth of substrate that still would need to be "seamed", primed, and painted ain't savin' you a dime.
Paint-wise, for Gain & improved contrast, consider RS-MaxxMudd LL as your best "easy" choice.
Spray-able or Roll-able
@ 140" diagonal (69" x 122") you'd achieve 13 foot lambert, just enough to have a respectably bright image w/1.4 gain. Might as well go for that size as dropping to 130" only gets you a 1 fl. increase.
Now if you go ahead and do a really righteous job on the drywall, then S-I-L-V-E-R would be the absolute BomB for you.
Take $60.00 away from the purchase of just a single sheet of WA-DW and apply it to this: http://www.gleempaint.com/noname.html
S-I-L-V-E-R consists of three ingredients:
1 Gallon Behr Faux Matt Glaze
1- 8 ounce tube of Liquitex Basics Silver
40 oz Distilled Water(ingredients can be halved)
2.0 gain minimum and that get's you 20 fl with that 'ol SONY
S-I-L-V-E-R provides fantastic Colors and appreciable Contrast boosting.
It's applied over a series of 7-8 finely applied "Duster" coats that are in turn applied over a carefully applied coating of a good quality White Primer.
But.......the wall surface (..and hence the primer coating...) must be smooth and blemish free. It must be sprayed.
......and you must want a screen that will be absolutely ready for that future upgrade to a $500.00 PJ.
Here are links to the application, which features S-I-L-V-E-R being utilized on a 16 "foot"
x 9 "foot"
original "Build" thread:http://www.avsforum.com/avs-vb/showthread.php?t=854134
AVS Members Pre-view partyhttp://www.avsforum.com/avs-vb/showthread.php?t=1159573