Buy double the amount you need, sure. Economical, not so much.
Grabbing another 'left-over' from a different batch CAN create an
imbalance. BTDT. Proper balance sheet eliminates that and *is* spec on
many commercial applications.
'Balance sheet' is inexpensive, does the job, meets spec.
You can use Balance sheet opposite GP grade or post-form grade.
Try to put some GP on top and post-form laminate on the bottom and
watch what happens. Again, BTDT.
Also, when seaming, make sure the seams are aligned top to bottom.
Ivan, you might check with a store that sells flooring tile. The
very tough paper is applied to a concrete slab, using mastic, and
then tiles are glued to it, rather than directly to the concrete.
This is called a "slip," and helps to reduce cracks in the
concrete inevitably showing through the tile above. The "slip"
I've seen is a pinkish or greenish color and quite inexpensive.
FWIW, I've had good luck just applying several coats of clear
finish to the reverse side of plywood to prevent warpage.
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