: I have some 1/8-inch ply it is not marine grade. It has warped within 2 days
: of purchase. Will marine grade resist warping better?

Not usually!

: Sage

Not a problem Sage.

Most plywood will warp a tad, whether you stand them up or lay them down. Sometimes it's the weight, if you lean them against a wall. Sometimes it's moisture balance if you lay them flat. The side that's exposed to more humidity will soften and the other side tighten..

If you want it flat, just turn it over and lay it down so the curve is up. It will lay down. I usually toss mine out on the lawn for a few minutes. Sometimes only 8 to 10 minutes is all it takes. Plywood is pretty forgiving. It doesn't matter what kind you get. Quite often they do the warping at the lumber-yard as they don't know how to stack it. They lay the units flat... Wrong! Most cabinet shops that I have seen or worked in put all the plywood ( good grades for finished sides ) standing on edge in narrow racks where they will be nearly verticle.

Rehd