When painting wet on wet, to avoid the paper buckling, it is a good idea to stretch the watercolour paper if it is 140LB(300 grams per square metre) or under in weight.
The fibres become stretched when wet and the paper will remain flat providing a perfect surface to paint on.
Watercolour paper
Board
Gummed parel tape
5 cms or 3.5 cms
Scissors
A dry lint free cloth
Clean sponge
Water