Because of the way that I paint with watercolour, I dont use masking fluid. One of the joys of using watercolour paint fo rme is that I can make a big mess and pull a painting out of it. My few experiences with masking fluid have not been fun because planning a painting out too much makes it too tight and regimented for me.
As you can see in this painting of Porthmadog Harbour, the whites are very fine and really stand out against the background.
This is by using Titanium white Winsor and Newton designer Gouache.
Because of adding it with a fine watercolour brush you will need to water it down slightly just be careful theat you do not water it down too much or it will go quite dull.
You can buy This Gouache Here: https://www.therange.co.uk/search?q=white%20gouache#sort=relevance&page=1&lpp=24
always add the Goauche at the end of the painting and make sure your brush is very clean before trying to use another watercolour paint or it will make the paint dull and dirty.