Masking Fluid
Saving whites in watercolour paintingsWatercolour painters apply liquid rubber latex to areas where they want to retain the white of the paper. After it has dried, they can paint over it, and no paint reaches the paper in the masked areas. After the latex is removed, these areas can be left white or overpainted as appropriate.
Masking fluid is an alkaline aqueous solution of rubber latex. It dries to a waterproof layer of rubber that adheres lightly to the paper, but can be removed without significantly altering the surface of the paper.
It is applied as a sacrificial layer to areas of watercolour paintings in which the painter wishes to retain the existing colour (often the white of the paper) while painting the surrounding region with in a free unfettered manner. When the layer of masking fluid has been removed, the painter can leave the area as it is, or continue painting it.
Transparent watercolour
Watercolour painting lends itself to application in thin, "transparent" layers (I'll explain why the word transparent is in quotes in a moment). The theory is that light passes through the paint layers, is reflected from the white paper, and passes back out through the paint again*. As it passes through the layers of paint, it is reflected off the surface of the pigment particles, with some light frequencies being preferentially absorbed at each reflection (so the light isn't actually passing through the pigment as it would have to be if the paint were truly transparent). The result is that the original white light emerges from its dual passage through the paint with a colour that is the complement of the colour that was most heavily absorbed by the pigment particles. In other words, if the particles absorbed mostly red light, the emerging light is green; if they absorbed mostly blue, the emerging light is yellow.
As the thickness of the paint layer increases, the number of particles that the light has to bounce off before escaping increases - and more light gets absorbed at each reflection. So adding more and more layers of "transparent" paint makes your painting darker and darker; you can't paint a region lighter with transparent paint. You can lighten it a bit with some of the more opaque paints (including white), but this has the unfortunate effect of turning the watercolour opaque - and the appearance of watercolour with added white is flat and lifeless by comparison with "transparent" paint alone. Similar comments apply to solid areas of opaque paint, as you'll discover if you try to paint out a mistake with white or gouache.
So for watercolour painters, the white of the paper is precious. It's the fourth primary colour (don't ever let anyone tell you there are only three primary colours!).
What does this have to do with masking fluid? Well, in order to get light regions in a wash, watercolour painters typically apply the paint in very thin layers (one of the reasons why watercolours that were painted with the old fugitive colours faded faster than oil paintings; there just wasn't as much pigment in them) to let the white paper shine through, and to get completely white regions, we leave the white paper untouched. This is called saving the lights or retaining the whites. But there's a technical problem. It's difficult to paint a smoothly gradated wash (sky, for example) around an area of white with an intricately shaped edge such as a mass of white flowers. The smoothly gradated wash calls for large brushes and confident strokes, but we're stuck carefully painting around the edges of the white region with small brushes.
Masking fluid to the rescue.
Masking fluid is s a solution of rubber latex which sticks to watercolour paper as a waterproof layer, rejects watercolour paint, and it has the wonderful property that it can be removed without damaging the paper once the paint has dried. We paint it over the region that's going to remain white, wait till it's dry and then apply the wash with those big brushes and confident strokes. When the masking fluid (of course it's rubber at this stage) is removed, the pristine white region emerges, with hard edges and as intricate a boundary as you like. Ralph's flyaway hair in Brown Study shows the sort of effect achievable with this technique. Note that the hair isn't quite as wispy as I'd have liked, because it's difficult to make lines of masking fluid as fine as hair.
Masking fluid smells of ammonia because latex is only soluble in alkaline conditions, and ammonia is a mildly alkaline. The latex rubber hardens quickly and is death on brushes. Once it's hardened on a brush you won't get it out. Never, never, never use your Kolinsky sable brush to apply masking fluid. If you use a brush at all (I don't) use a cheap acrylic brush, and moisten it with dishwashing liquid or soap before use. But the fluid dries faster than you think, and you'll get involved in applying it, and inevitably, you'll lose brushes. I've found a steel-nibbed pen to be quite a useful alternative. The masking fluid does harden on it, but it's robust enough for me to clean it off without damaging the nib. And another alternative that I've found useful is the solid blenders that have become available. They're solid in the sense that they are a single block of material (silicone?) rather than a brush, but they're soft enough to be a bit responsive when you're pushing masking fluid around.
Don;t paint masking fluid onto wet paper, the water will carry it into the fibres of the paper, and you won;t be able to get it off. And while we're on the subject, thick layers of masking fluid are easier to remove than thin ones, which stick like the very devil. You may want to add more over the top of the first layer if that layer is very thin.. Once the masking fluid is on, wait till it's dry (10 to 20 minutes, depending on thickness). If you paint over it before it's dry, you'll risk ruining your brushes, and it's difficult to predict whether the paint that's mixed in with the mask will come off with it, or stain the paper.
Once it's dry, paint your other wash or washes, wait till they are dry, and then remove the mask by rubbing gently with your finger tips. Although actually, you can be pretty rough with it. I've raised blisters removing large areas of masking fluid from rough watercolour paper. Alternatively, if the layer is thick enough, you may be able to peel it off, and you may be able to remove it with a kneadable eraser, or in fact, by rolling a ball of previously-removed masking fluid over it.
In its raw state, liquid latex is clear, and it can be difficult to see which areas you've masked. So the manufacturers kindly add a little pigment, often a pale yellow. This is meant to come off with the mask, but you may find that it stains the paper a very pale yellow. Sometimes this matters; more often it's not a problem.
Advanced techniques
The masked region doesn't have to remain white. You can paint over it later. In my painting Spring (Port Wine Magolia), the branches and the magolia bud were masked while I painted the background. In the grassy region at the bottom half of the painting, I painted the bright green blades at the front first. Once they were dry, I masked them and overpainted them with some blades that crossed behind them. After repeating this paint-mask-overpaint process two or three times, I was left with a grassy tussock in which successive overpaintings had added more and more of the background grass, without having had to carefully cut in the dark background blades around the light foreground blades. At that stage, all the grass-blades were masked, and I could add an unstructured dark background wash with impunity. When the mask came off, a little of the paint I'd applied to start with came with it, but that was easy to touch up without damaging the overall appearance of the grassy area.
I then followed a similar multistage process to cope with the multiplicity of small buds on the branches - masking those while I painted the majority of the bark, then revealing them and painting the chiaroscuro for each bud individually.
Other paintings that show what can be done with masking fluid (and a good deal of patience) are the reflection paintings in my Rocks in the River series. The reflections in these are retained by painstaking application of the masking fluid with a steel-nibbed pen, loaded with masking fluid, held at the extreme tip and bounced on the paper rather in the manner of a drummer doing a drum roll. A stippled pattern of dots resulted that I slowly built up into the reflections on the water.
By the way, there's no Black paint in those paintings. The deep, deep black is achieved with washes of Quinacridone Magenta/Winsor Green (Blue shade).
*This theory is slightly simplified, in two respects. First, some of the paint particles (especially in paints with small pigment particles) are absorbed into the paper, and some of the light penetrates into the paper, so the description "passes through the paint and is then reflected by the paper" unrealistically separates out two effects that are mixed.
Secondly, the transparency of the paints differs. Some paints such as Burnt Sienna and Ultramarine are highly transparent, whereas others such as Lemon Yellow and the Cadmium colours are fairly opaque.
But much of the time, it's reasonable to think of the paint as layers of transparent colour, with a white "mirror" underneath.
