flatten thin glued up panels

I have some thin (1/2" or less) glued up panels that have cupped. In the past I've applied water to one side and they've straightened out. However I can't remember if I apply that to the concave or convex side. I thought it was the concave side but my googling suggests that I was wrong. Should the water be applied to the convex side instead? (I need to take better notes when stuff works.... (which isn't very often) )
Thx.