It sounds like your cardstock is saturated if it's buckling that badly so maybe you're a bit heavy-handed with the mister? either that or it's very lightweight cardstock. Adding moisture is almost inevitably going to make cardstock react but it should flatten back out well enough to stick to another layer and look smooth.
Try misting more lightly and letting it dry and then if you want more saturated colour, add another light misting.
You could also try ironing your buckled cardstock to resuce it. Put it between a couple of sheets of clean copy paper and use a fairly hot iron.