Aspects of tap water that are a concern, when adding water to acrylics to thin heavy-bodied acrylic paint are pH, granulation (from microbes), and hardness. D.I., or distilled/purified, water is pH neutral. Most tap water is in the pH neutral range, but unless you have pH strips; there is no way to be sure from one day to the next. If you are going to purchase something for your paint – purchase a water filter and double filter it or just buy a gallon if purified/distilled water. Changes in pH can alter the paper painted on and ruin a beautiful painting.
The next concern, and one of the biggest, is whether your water source has microbial entities. These microscopic critters can change your paint over time/as it dries and even digest your paint and turn it another color! Oh, the humanity! Nothing is worse than microbe dung interfering with your final painting. Can we find a source of purified water? The preference for a smooth body of paints, once dried, is to make sure it is not granulated from microbial interference.
I have heard some artists say, “Using the local water is like having a piece of that area on my painting.” That’s one way to look at it. Another way to look at it is the following: tap water from copper or lead pipes can turn your paints from consistently vivid to orange or green color shifts. If the local tap has sulfur – then your paints will display other properties. These issues can also damage the paper or canvas used by an artist. Then again, some paint brands are expensive; let’s stick to purified/distilled water. Investing in quality water to go with the money spent on paint will be an investment in the life of your painting.
Keep in mind, acrylic paints have a resin portion. Any water used to thin paint is eventually going to evaporate. In humid environments, anything less than pure water can mold your paints! Take it from me, when your paints cure; you want them to look as close to your wet paint as possible. The opposite spectrum is a very dry environment. Water will pull out of your paint first in this environment. Any left-over products in the water that don’t belong will damage the painting in ways that are not welcome (think cracking or crazing). The best way to help your painting from cracking (thick cracks in the dried paint) or crazing (wrinkles in the paint) in a dry environment is to either 1) place a cover over the top of the painting, or 2) spritz the back of your canvas with some of that distilled water (or both).
Acrylics need a good drying time before they are ready for clients or even for hanging on the walls. Place them face up, cover the top in a dry environment the first 24 hours (place it in the oven – but be sure it is NOT ON!) OR in a humid environment place the finished canvas on a drying rack of some sort (something to allow air flow under the canvas – open the oven and grab one of those racks and put it on a level, elevated surface). Once the paint is dry to the touch, then the painting can cure properly over the next few weeks.