(No) buckle up
In order to help prevent your watercolour paper from buckling, you should use at least a 140 lb cotton rag or better. But the main thing, no matter the paper, is to wet it on both sides.
This doesn't tend to work so well with 90 lb papers because they just can't take the water load. So, if I do happen to find myself possessed of 90 lb papers, I save them for really light things, or printmaking of super thin layers.