Which watercolor paper resists warping for wet-on-wet painting?

For wet-on-wet that stays flatter and go with 140lb 140lb since the cotton fibers and 140 lb weight handle heavy floods and multiple layers much better than wood pulp options. It is a gummed pad so pages tear out cleanly while keeping a nice edge, and the format matches your preferred sizes.

Been there... Another vote for 100 percent cotton at 300 gsm or higher for true wet-on-wet. Cold press takes floods without the paper turning to waves, especially if the pad is gummed on one edge so you can pull a clean sheet. If I know I will stack layers, I mist the back lightly and tape the margins to a board while it dries. Keeps it flat. Then tear out when you are done and the edge stays neat.

One more angle which, yeah pay attention to grain direction and place the long edge along the grain so the sheet curls less during big wet-in-wet washes. Keep the sheet horizontal while drying and if it domes, flip it and mist the back very lightly to counter the memory. Pads that use a clean tear design look tidy in 9x12 and 12x16 and no deckle fuss.

I paint big skies and soak the page and the only times I fight warping are with wood pulp sheets. Cotton handles the flood then rebounds as it dries, especially if I dry flat instead of upright. For tidy edges I prefer pads with microperfs since they release cleanly. Works great.

For clean tear-outs, choose microperforated gummed pads in 9x12 or 12x16, 100 percent cotton at 300 gsm with both internal and external sizing so it handles multiple wet layers without pilling. If budget allows, go 640 gsm for big washes and minimal movement.

If your goal is heavy glazing the sizing quality matters as much as weight and well sized cotton lets you lay down multiple wet passes and lift without scarring where cellulose tends to rough up and then buckles more with each layer.

Cold press in 9x12 or 12x16 hits the sweet spot for wet-on-wet control, and pads with a single gummed edge release with a nice straight tear.

Quick trick if you must stay at 140 lb cotton & still want flat results. Lightly wet the back before your first wash and tape all four edges to a rigid board, paint your layers, then let the whole thing dry completely before removing the tape and tearing the sheet from the pad. It evens out the pull so you get minimal cockling, and you keep that clean edge you want. 💡

On a budget I use cotton blend sheets at 300 gsm and clamp them on a board with wide tape and binder clips, honestly then flood away, and yeah there is a little wobble while wet but it dries flat enough which, yeah when I need perfect flat I borrow a heavier 640 gsm sheet. For your sizes the microperforated pads pop out cleanly and save you from trimming.