After installing the deck lines, I saturate the line with varnish at the hole locations. This prevents wicking. After the varnish is dry, I coat the rope-skin junction with thinned Marine GOOP. A flexible sealant is necessary, since the lines will be pulled and stretched. Other sealers will also work as long as they're UV resistant, but I would suggest that you NOT use silicone. Silicone sealers are relatively weak bonding and the residue they leave behind makes it difficult to get anything else to bond to the area.