The best formula believed to have been designed so far for an anti-fouling paint for the bottoms of steel ships or boats is based on ferric oxide in very fine division with a portion of zinc oxide, arsenic and red oxide of mercury in a vehicle of an alcoholic solution of shellac, with addition of pine oil. Linseed oil or fat varnishes must be avoided, as these promote marine growth in spite of the presence of poison. Anti-fouling paints must be lighter in gravity, than the non-corrosive paints over which they are applied, yet must not act as solvents for the latter.