Testing scale buildup in a well tool can include pressurizing separate cationic and anionic solutions, then heating the separate solutions, mixing together the solutions, and exposing the well tool to the mixed solutions. A well tool can be constructed by a) determining velocities of flow at a predetermined offset from surfaces of a geometric model representative of the well tool, b) calculating scale buildup on the surfaces based at least in part on the velocities, c) reducing pressure change per unit distance along selected ones of the surfaces having greater than a predetermined level of scale buildup, and d) repeating steps a-c until all scale buildup is no greater than the predetermined level. A method of predicting scale buildup can include inputting a parameter indicative of flow through a well tool to a mathematical model, which determines a rate of scale buildup on a surface of the well tool.