3.6.2.1 Sanding 



This method is probably the most common used by the owners of small 

 pleasure craft because of the low cost involved. Where wet/dry or water 

 sandpaper is employed, the removed paint film can be washed into the 

 public sewer or ground water. This paint removal practice may occur well 

 inland of the waters edge if the boats are small enough to be put on a 

 trailer and stored at a residence. 



3.6.2.2 Sand or Grit Blasting 



This is the method of choice in commercial yards because of its removal 

 speed. Because of the size of the vessels involved, much of this work is 

 done at or near the water's edge. Alternatives to grit blasting, 

 investigated by the Navy, include cavitating high pressure water jets and 

 high energy beams. In all types of surface preparation, the old paint, 

 rust, and marine organisms are found mixed in the spent blasting media or 

 liquid. Surface preparation methods comprise dry abrasive blasting, 

 hydroblasting, wet blasting, water cone blasting, and chemical paint 

 stripping. Surface preparation methods, other than dry blasting, are not 

 common in the industry. Wet blasting and water cone blasting is confined 

 principally to Navy ships having special coatings including TBT. Chemical 

 paint stripping is rare and is used only on small localized areas made of 

 more delicate materials. 



Dry abrasive blasting (sandblasting, grit blasting) , is the most common 

 method of surface preparation. When employed, spent abrasive is the 

 principal source of solids in the drydock discharge. Particle sizes of 

 the used grit range from fine dust to whole bits of abrasive, 

 approximately one-eight inch in diameter. Some of the spent grit falls 

 directly into drainage gutters, especially if a ship is large and the hull 

 sits over the drains. The potential also exists for the abrasive to be 

 washed into the drains from storm runoff, shipboard wastewater discharges 

 on the dock, hosing, seepage, or other sources of water to the dry dock. 

 The spent grit is for the most part, settleable. In dry dock areas, it 

 can be vacuumed up for proper disposal. Sometimes, sand is used as the 

 abrasive, instead of utility slag or copper slag. Delicate equipment, 

 such as sonar domes, are occasionally sand blasted. Muminum-clad hulls 

 are often blasted with sand instead of grit to minimize metal erosion 

 during blasting. 



The major pollution problem from hydroblasting (U.S. NAVSEA, 1984) is that 

 the volumes of water used increase the potential that the paint and grit 

 will be flushed into the drainage discharge. Since oxidation of the sur- 

 face of the hull of the ship will prevent a good bond between the fresh 

 paint and metal, rust inhibitors, which contain compounds such as sodium 

 nitrite and diammonium phosphate, are used. However, hydroblasting is not 

 preferred by ship repair facilities, because the resulting surface 

 obtained is not as suitable for paint adhesion as the surface obtained by 

 dry grit blasting. 



111-22 



