-11-9.SHOP PRIMER FABRICATION SYSTEM1) Advantages of the systemSince Japanese shipbuilders introduced the block building production system about fifty years ago, shop primers have played a very important role.In all cases, shop primer is coated onto the steel surface which is first cleaned by abrasive blasting at the steel mill or shipbuilding plant.After coating the steel plates with shop primer, the plates are cut and welded to make blocks. The blocks are then assembled to form a ship in a dry dock. During this period of shipbuilding, the shop primer film protects the steel surface from unnecessary corrosion and minimizes the surface preparation costs required before successive painting.Moreover, the shop primer fabrication system has enabled shipbuilders to shorten their building schedules and reduce building costs.The advantages of shop primers also apply to the fabrication of plants, bridges and other steel structures.2) Film thickness of the shop primerAs shown on Table 6, phenolic resin modified wash primer and organic and inorganic zinc rich primers are used as shop primer.The recommended film thickness is from 15 to 20 microns for wash primer and from 15 to 30 microns for zinc rich primer.The roughness of the blasted steel surface is so great that a thin film of shop primer does not evenly cover the surface. To ensure the proper film thickness of the shop primer, cold-rolled steel plates with a smooth surface are put on the blasted surface as reference panels. After coating, the avarage thickness of these panels is determined using an electromagnetic thickness gauge.Because the weldability of the steel plate is affected by the existence of the shop primer film, the film should be kept as thin as possible to the extent that sufficient anticorrosiveness is ensured.Sometimes shop primer is coated very thick on steel plates for long-term corrosion prevention. In this case, the shop primer film around the parts to be welded must be removed by a power tool before welding.3) Transportation and storage of shop-primed steelsThe shop-primed steels or plant equipments are often transported over the sea and stacked portside or at a work site. During transportation and storage, shop primer films are exposed to very severe conditions.Much salt deposits on them and rainwater or condensed water penetrates the stack of the steel.The shop primer film rapidly deteriorates in the presence of water, oxygen and salt. For example, the zinc powder in zinc rich primer quickly loses its galvanic anticorrosiveness. Within a few weeks, heavy rust appears on the steel.To keep the shop primed surface in good condition, all the steel must be kept dry and free from contamination by salt from the sea or salty soil.Steel coated with phenolic resin modified wash primer will remain protected for 3 to 6 months. Steel coated with zinc rich primer remains corrosion-free nearly a year in normal weathering conditions.4) Compatibility of paints with shop primersInorganic zinc rich paint can be applied only on inorganic zinc rich primer or blasted steel surfaces because of its poor compatibility with organic primers.Oil primers, alkyd modified oil primers and phenolic primers are not recommended to apply on to organic and inorganic zinc primers as the oil component in these primers will react with the zinc and produce metallic soap which will deteriorate the adhesion between the zinc rich primer surface and overcoated film.Table 6 shows the compatibility of paints with shop primers.