Your watercolor could be cracking because of shrinkage, the use of low-quality or inexpensive watercolor brands, the high water content in tube brands, the use of poor-quality ingredients, unfavorably hot climate, or poor storage.
The moisturisers commonly used in watercolours can include honey, as such, you could try to add a drop of honey to your paint. The plasticiser used in watercolour is glycerin, so again we could try to add a drop of this. Finally, we could try adding a little more of the binder, gum arabic.
If your colour has solidified in the watercolour tube, it is not possible to return it to its liquid form. But it can be used as a type of pan colour. Simply add a little Winsor & Newton Gum Arabic medium to water and then use the dried tubes as pans.
Repairing cracked watercolors
I would recommend using a brush to apply the water and swirling it around a bit, just like you would when painting. As a result, the paints are activated and forged back together. Let the watercolors air dry for a bit and they will look as good as new.
Some watercolours will crack a bit but not so much that they fall out. Next time you fill them, add a touch of glycerine to overcome this. Some manufacturers use honey to keep the paints moist - such as M. Graham and Sennelier.
Pan watercolors should be good for at least 10 years. This varies depending upon the storage conditions. Moldy watercolors are usually discarded. It may be possible to salvage tubes that are dry or separating.
Since watercolor from a tube comes out more vibrant, getting the same color with paint from a pan will take more paint and less water. In the above photo, the same color was applied to paper with a tiny amount of water. As you can see, the watercolor from the tube is distinctly more vibrant.
Use a dropper, mix thoroughly and wait 1 day before adding more. Add equal drops of glycerine and water. Adding too much will prevent it from drying if applied thickly. How much to add depends on pigment.
Watercolor painting has a reputation for being unforgiving, but there are several different ways to fix mistakes in watercolor, make changes, or even to incorporate mistakes into your painting if you can accept some as "happy accidents." You can blot paint up while it is still damp, lift out paint once it has dried, ...
Aerospray Fixative - Use this universal spray fixative as a final fixative for charcoal, pencil, pastel, watercolor, gouache, art prints, ink-jet prints, and photographs to protect from fingerprints, humidity and dust. There is no color shift when applied in thin layers.
How it works: A sanded eraser is just what it sounds like, an eraser with some grit to it, not unlike sandpaper. Sanded erasers are very aggressive erasers. They can help rub away color, and even the top layer of the paper to recover some white underneath stained paper fibers.
Because Cold-Press paper offers the best of both worlds, it's the paper of choice for many professional watercolor artists and it is the one I recommend beginners look for.
WORK FROM LIGHT TO DARK
With watercolor it's important to lay down your light colors first and work towards the darker colors. Have patience - there's no rush. We start with the light colors first because once you lay down the dark colors, it's hard to undo.
Always give your watercolor painting about 5 to 15 minutes to dry for each layer. If you're in doubt that the painting hasn't dried properly, you can always check the surface temperature.
Honey allows high pigment loads and contributes to smooth washes. "Honey draws moisture from the air so our color always remains moist (sticky) in the watercolor palette even when exposed to open air year around. It does not get rock hard on the palette or in the tube like other brands.
Honey allows for high pigment loads, and an incredibly smooth application. The combination of gum arabic and honey result in a binder that is free from additives and filler, allowing for the purest painting experience possible.
Honey also keeps the paint from getting rock hard on the palette, and the moist, slightly sticky hygroscopic influence of honey gives body to the paint making it easy to work with. Honey too has a way of blending and evenly dissolving the pigments for smooth washes of color when painting.
Watercolour can deteriorate over time. More specifically, the sizing of the watercolour paper can deteriorate. Most watercolour paper are sized with gelatin either internally or externally. The sizing allows water or paint to sit on the surface of the paper rather than soak into the paper fiber.
Watercolor paint is more expensive than acrylics or oils because they contain a higher concentration of pigment, and they often require more processing. A small tube of watercolor lasts a long time, so that balances out the higher price.