Depending which paper you use, it can be difficult to remove, and when you do remove it, the paper must be totally dry. Often I choose not to use it at all, preferring to paint around shapes, and allowing paint to bleed into the areas, which can allow for a softer end result. But when you are wishing to preserve small shapes, in order to have more freedom with the washes, then some masking fluid may be the right choice.
So, back to brands: I like Windsor & Newton, because it doesnt rip the paper ( providing the paper is robust enough ) Recently though I have been using the Masquepen from Cruddas Innovations Ltd, which is available from Jackson's Art Supplies UK, as it has the option of a very fine nib for applying extremely fine lines. You must be assiduous in cleaning out the nib in order to prevent blockages.
I have had trouble with this, and wrote to the company, who very kindly supplied me with more nibs and needles to clean with. The fluid is very easy to remove, not damaging to the surface of the paper, so, worth experimenting with.. so give it a try!