Adding water dilutes the binder (glue quality) that makes the paint stick. Adding 10% water or less will maintain adhesion. In some fine art applications such as watercolor technique or some fine art airbrushing, (where there is no abrasion, washing, or wear and tear), the amount of binder is not as important. If you are painting a wall, a sign or clothing, you will want the binder to remain strong. If you mix any of the clear mediums, varnishes or gels with your color instead of water, the adhesion will remain strong. Adding a clear topcoat over your finished work will also protect any paint layers below that are weak in binder.