-8-7.EFFECT OF COATING TIMES AND FILM THICKNESSA coat of paint inevitably bears some defects and a partial deficiency in film thickness even when the paint is applied thickly. To reduce these defects, it is recommended to apply two or more coats of rust preventive primer before the topcoating.The thicker primer film, if it bears no defects, is more rust preventive.Table 2 shows the durability of paint film in various weathering conditions.These paint films consist of oil-based rust preventive primers and alkyd top coats having a total film thickness of about 125 microns.In the painting and coating manual of the Royal Dutch/Shell group, the coating intervals for the maintenance of their paint are as shown on Table 3. These intervals have been decided for the standard painting system which consists of 2 coats of red lead/red oxide oil alkyd primer and 2 coats of aluminium paint.The total thickness of the film is at least 125 microns.ConditionStructureTemperate ClimateTropical orSemitropical ClimateExposure to Salt-Bearing WindsExposure FrequentSandstorms andMarine AmeosphereStorage tanks Walls RoofsPipes & other structures10 - 12 years7 - 10 years8 - 10 years8 - 10 years4 - 6 years6 - 8 years4 - 6 years3 - 5 years3 - 5 years2 - 4 years2 - 4 years2 - 4 years[Manual, Painting and Coating DEP30.48.00 10-Gem. Jan. 1973 (Royal Dutch/Shell Group)][Surface Preparation: SSPC SP 6 Commercial Blast Cleaning]Table 3 Repainting intervel for PlantsConditionAverage Repainting IntervalSeasideIndustrialRuralMountain region3.9 years6.0 years6.9 years7.8 years[Report of Japan National Railway Technical Research Institute No.892 (1974) Feb.][Surface Preparation: SSPC SP 10 Near White Metal Blast Cleaning]Table 2 Weather Conditions and Durability of Paint Film