Stained seams have been observed on finished linings. They range from small dark spots in the seam center to dark patches which appear to emanate from seams.
Causes: Seams become stained due to water passing through the liner either during cure (hot water passing out) or after cure (groundwater passing into the liner). When hot water passes out, spots or stains are sometimes caused by impurities being filtered out of the water. Also, if water can find its way under the seam tape, it may track along the seam causing impurities to be filtered out in the seam area for some distance.
If seams become damaged during installation it would then allow water to pass out during cure and back in from the ground following the draining of the curing water.
It is also possible for resin to find its way directly underneath the seam tape and under certain conditions, to cure in such a way which causes small holes in the covering tape to melt along the edge of the white tape.
If the liner is porous due to poor impregnation or monomer boil then at a later date the water may be able to find a path from the groundwater to the inside surface of the liner depositing oxidized iron/manganese salts on the the surface of the coating.
Figure 1: Small dark stains on seams. (Click photo to view video.)
Figure 2: More extensive stain. (Click photo to view video.)
Figure 3: Severe stains. (Click photo to view video.)
Solutions: Stained seams can be avoided by preventing damage to the seams by carefully handling them during impregnation and site installation. Any leaks on the seam, or anywhere else for that matter will be clear and repaired during impregnation.
If the lining is undersized by too large of a margin, excessive stress may be placed on the seam causing it to develop holes in the white tape under the cover strip. As an example see Figure 1 above.
It is important to impregnate the liner at the correct nip roller setting and vacuum to help avoid porosity from the air being drawn into or remaining in the liner.
Cure of the liner should not be too rapid to avoid overheating the seam area.