<p>Inorganic fouling is a challenging problem in heat exchanger applications, particularly for compact heat exchangers under high-temperature conditions. Surface treatment is an effective approach to mitigate the fouling of hard water flow in heat exchangers. In this study, the effectiveness of an anti-fouling treatment on the inner surface of metal three-dimensional microscale hollow structures (M-3D-MSHSs) was verified by using very hard water at a water temperature of 60&#xa0;°C and a wall temperature of 70&#xa0;°C; additionally, the location, amount of attachment and composition of the impurities in the test circuit were investigated. The effects of the measures against the deposition and adhesion of impurities using very hard water on the test pieces (TPs) were evaluated. The pressure loss of the sample without coating sharply increased and reached 4 times the original pressure drop after the 2-week tests. However, the variation in the pressure loss of the sample with a coating was negligible even after 5&#xa0;weeks of continuous tests. Here, the coating surface of the test sample had the ability to suppress the precipitation of the impurity particles from water with the coating treatment on the rough wall and the adjustment of the zeta potential of the adjacent wall surface. Finally, an anti-fouling technology roadmap that considered cost and feasibility to suppress the precipitation and adhesion of impurities from very hard water with heat exchanger channels was recommended.</p>