Abstract: A method and an apparatus for coating tanks and pipe systems internally in that, first, a tank (1) is filled with a liquid (3) consisting of water to which is admixed an acid (10). Oxide coating on the internal surface is removed through heating and circulating the liquid (3) through a filter (4). The liquid (3) is neutralized through the admixture of a base (12). Approximately one fifth of the neutralized liquid (3) is drawn off, the tank (1) being refilled with a concentrated metal solution (15). The temperature, acidity and metal concentration of the liquid (3) are maintained close to constant through supplying heat, acid (10) or base or base (12), and concentrated metal solution (15), respectively. Air or vapor is supplied through a blowing pipe (5) and creates stirring, surplus liquid and gas being drained through a pipe (16). When the internal surface of the tank (1) has received a coating having the desired thickness, the process is interrupted in that the liquid (3) is cooled and drained.