Based on artist's input and distributors, we find one of the better masking fluids for watercolour paper is Schminckle White Liquid Frisket. With the Schminkle White there is reportedly no tearing when used.
Here's a very detailed and helpful article on masking fluid used for watercolour paper. https://www.jacksonsart.com/blog/2013/02/22/comparing-masking-fluids/
I would recommend stretching the paper beforehand just as with Stonehenge White to minimize the risk of warping, buckling, etc.. This is especially true with lighter paper.
You can avoid stretching with the heavier paper (140lb, 300lb, etc.) by simply taping the paper to a board before painting as well.
With the Schminkle White there is reportedly no tearing when used.
Here's a very detailed and helpful article on masking fluid used for watercolour paper. https://www.jacksonsart.com/blog/2013/02/22/comparing-masking-fluids/
Does black watercolor paper need to stretch first?
You can avoid stretching with the heavier paper (140lb, 300lb, etc.) by simply taping the paper to a board before painting as well.