What's the best way to preserve white areas in watercolour?
7 tips for working with masking fluid, from Artists &amp; Illustrators

I use a great deal of water, sometimes pouring colour onto the paper from a small jar and then responding to its progress with further washes as it begins to dry.

I normally apply masking fluid to protect small, relatively complex shapes.

Masking fluid is often a difficult medium. sometimes to the extent that artists give up on it. However by following these tips  the ‘difficulties’ can quite easily be overcome.


Plan thoroughly. To get the most out of masking fluid, it should be considered very carefully at the planning stage of a painting. In most cases it is applied before any paint comes into contact with the paper.
The marks made and therefore consequently left by the masking fluid on its removal have to be carefully appraised. It might help to think of it not as masking fluid but as ‘white paint’. I considered the placement of paint but has taken a less than considered approach to the application of the masking fluid. A badly planned and poorly applied area of masking fluid can have as damaging effect to a painting as a poorly painted passage and can leave the painting in ruins.
Dilute to taste. Through experience, I have learnt that sometimes masking fluid can be too thick if used straight from the bottle. I often water it down to aid the flow of the masking fluid onto the paper, which is useful for creating finer lines or more finely spattered .
Test the masking fluid on some sample scraps of your intended watercolour paper. If the paper used is a soft paper, the masking fluid might pull at the paper when it is removed. It might be advantageous to dilute the masking fluid with water, as this would exert less of a pull on the paper.
Handle with care. Never shake the bottle up.
Don't work wet. Make certain that the paper is thoroughly dry before applying the masking fluid, otherwise you run the risk of the masking fluid penetrating the top layer of the paper. This could result in the paper being torn when the masking fluid is removed.
Always leave the masking fluid to dry fully before over-painting too. To check this, carefully touch the masking fluid with your finger tip. 
Remove it carefully. Some artists use a soft rubber to remove the masking fluid from the paper. I remove it by gently rubbing with my fingertips.