On exterior duct surfaces, insulation is installed by impaling it on weld pins and securing with speed clips or washers, or using special weld pins with integral cupped head washers. Unfaced boards can then be finished with reinforced insulating cement, canvas, or weatherproof mastic, or with sheet metal, depending upon the application. See Figure 28. Faced boards can be installed in the same way. Joints between boards are sealed with pressure-sensitive tape or glass fabric and mastic.