Masking fluid is a liquid rubber solution that is applied to the watercolor paper to save parts of the white paper. Small white details are difficult to save by just painting around them. Then masking fluid is the solution. You simply paint masking fluid on the area that should be white. After the mask has dried, you paint watercolor paint over and let it dry, then you rub off the mask, the result is white details in the places that you previously painted with the masking fluid.
Of course it is also possible to use masking fluid on already painted areas: Example: Paint a yellow surface, when it has dried put a few dots with masking fluid, let it dry and paint green over the yellow, rub off the mask and you have a lawn with dandelions.
Personally, I prefer masking fluid in a bottle, for me it should be white, not blue or gray. There are various pens and pins with masking fluid as well. In my experience, they work doubtfully, It’s better with the liquid in a bottle, then you can choose which tool you want to use.
Using a brush
Do not use your finest brush for masking. It can be destroyed! Find an old cheap synthetic brush. Preferably of smaller size. You can protect the brush by dipping it in soap or washing-up liquid (do not do so with a nice brush) it prevents the masking fluid from getting stuck in the straws.
Do as follows: Dip or put a drop of the soap on the brush straws, rub in the soap with your fingers so that all the straws are covered. Feel free to pull off excess soap with your fingers, it does not take much, and we want to affect the masking fluid as little as possible.
When you have finished masking, the brush should be washed immediately in water, if the liquid is allowed to dry, the brush is ruined. There is no solvent for dry masking fluid.
Using a Color Shaper
A Color Shaper looks like a regular brush with a silicone top instead of brush hair. It is a useful tool for the watercolor painter, among other things for masking fluid. Dip it into the liquid and apply to the paper, it’s really easy. It does not even need to be cleaned, just let it dry and then roll off the dry masking fluid with your fingers.
Using a dip pen
When you need to save out very fine details, a dip pen is an excellent tool, a dip pen is good for watercolor painting in general, fine lines that are difficult with a brush are easy to do with a dip pen.
The masking fluid is quite viscous, you need to dilute it with water for it to work with a dip pen. If you dilute too much, it becomes difficult to get rid of when it is dry, if you dilute too little, it does not work with the pen. It’s difficult.
Here’s how I do it: Dip the pen in water and then directly in masking fluid, then dip the pen in water again, it usually works for me.
When you have finished masking, you should clean the pen, remove the nib from the holder, wash it in water (thoroughly) if you have dipped the pen so deep in the mastering liquid that the pen holder has also been dipped, you should also clean this in water.
Using a toothbrush
A trick that I sometimes use is a toothbrush, it is good to sprinkle and drip with. It is also possible to sprinkle masking fluid.
Dip the toothbrush in masking fluid, then let most of it drain off, otherwise you will get large blots on the paper. Hold the brush with the straws upwards, pull with one finger in the direction from front to back – towards yourself (not in the other direction, in which case you yourself will be masked). You will achieve splashes on the paper, larger dots in the beginning, smaller as the toothbrush has less masking fluid.
Toothpicks are a common way to apply masking fluid, or the brush shaft. you can certainly come up with more methods. If the method works the way you want, it’s good. There are those who say that it is cheating to use masking fluid, I say: cheating does not exist when it comes to watercolor. Everything is allowed!