my guess, because massa paper its intentional use was most likely NOT for backing (huge) posters
just wondering, you are going to start linnen backing and start immediately which something that large that needs cotton duck and massa paper overlays ...
so may i ask what are you planning to back ? (24 sheet billboard
)
in my humble opinion i would try a few small (and preferable worthless) poster in case something will go wrong
(and believe me something will go wrong , or at least there's a lot of room for improvement after your first diy backed poster)