: I made my strongback, ala Ted Moores, out of ordinary 3/4" construction
: grade plywood. My strongback is straight, strong and stable. Construction
: grade plywood should work just fine.

i made a 3"x6" scarf jointed, glued, screwed(every 6in) and tatooed interior strongback out of 5/8" lower grade ply. and kept noticing deviations of 1/16 - 1/8" every next day after true up the evening before. (i use internal stringlines always there to check - whew)

so frustrated after 3 days i took it all apart and dismantled the s/b and when bending/investigating ea pc by hand i noticed a soft point in one pc, broke it right there and one of the internal veneers had a clean crack right across it. remade the internal to 5"x6" shaped and i still don't think/feel it's reliable.

however, might have been that one pc in a million. i'll still use lower grades, but if the ply looks bashed up a bit, i will beware for reliability.