I'm going to the seaside at the weekend, and I plan on using my watercolours while I'm there as a way to relax. I have these brushes that can store water in the end of them so I don't need to bring a water pot. However, if the water runs out, I want to know if it's okay to refill them with water from the sea. That way, I don't have to worry about bringing extra water with me. I'd like to know if using the sea water would have an effect on the quality of both the paint and if it'll damage the brushes.
Yes, you can use salt water with water colours. But remember, table salt (or salt water) is highly hydrophilic. Salt attracts water. Anywhere salt is present on your paper it will attract water from the area. The water colour pigments floating in your solution will concentrate around the salt particles. As the water is drawn closer by the salt you will get concentrated areas of pigments in those areas where salt particles are present. So if you are looking to achieve a spotted texture on the surface of your water colour painting salt is a good choice.
As an experiment try sprinkling some table salt on the surface of your water colour painting, the sprinkled salt will create a more pronounced effect as the larger salt crystals in the table salt will attract a larger amount of water.
When done painting clean your brush with regular tap water. This will keep a layer of salt from precipitating out on the hairs of your brush as the salt water dries.